The following contains spoilers for Episode 3 of Lord of the Rings: The Rings of Power. Amazon's The Rings of Power has been introducing the audience to a lot of Middle-earth lore that might be unfamiliar to them. As the events of the show cover a time period long before the events of The Hobbit or Lord of the Rings, only the most die-hard Tolkien fans will likely be familiar with the names and places of the Second Age. Luckily, the show does a pretty good job of introducing those new ideas to the audience in a way that isn't confusing, though some of the dialogue can become a bit exposition-heavy because of it.
In Episode 3 of The Rings of Power, the character of Elendil is introduced, as his ship is the one that saves Galadriel and Halbrand from the sea. When he takes them to the island of Númenor, it becomes clear that he is important to the kingdom, and there are hints that he is a character that will be influential in the grand scheme of Middle-earth later on. Who is Elendil, and why is he important to Tolkien's world?
In order to understand Elendil's story, there needs to first be a brief recap of Númenor's history, and what becomes of the great kingdom. Because of the time period Rings of Power takes place during, it's likely that the show will eventually cover this, likely at or near the end of the story. Númenor was an island that was brought up from the sea by the Valar as a gift to Men after the war against the Dark Lord Morgoth. The Valar banned the Númenóreans from sailing far to the West because they did not want them coming across the Undying Lands. The Númenóreans began to resent this because they felt that their chance at everlasting life was being denied, so in an effort to take their power back, they began to sail to the east and colonize parts of Middle-earth.
In year 3255 of the Second Age (SA), one particular king of Númenor, Ar-Pharazôn, sailed to Middle-earth, only to have his mind corrupted by Sauron. Because of a combination of Sauron's influence and Ar-Pharazôn's own fear of aging and death, he sailed West with the intention to wage war against the Valar and take the Undying Lands for himself. The Valar didn't take this lightly, of course, and they called upon Ilúvatar to change the shape of the world. Aman and Tol Eressëa were wiped from the world altogether, which changed its shape from flat to round. Númenor was sunk beneath the sea, which killed its inhabitants, including Sauron, who was then robbed of his ability to assume different forms.
Elendil was the son of Amandil, a Lord who was one of the Faithful - aka the Númenóreans who were still loyal to the Valar. They were friends of the Elves and were against the practices of Ar-Pharazôn and Sauron. Elendil had two sons, Isildur and Anárion (his daughter Eärien who appears in The Rings of Power was invented specifically for the show). The name Isildur will definitely be familiar to any Lord of the Rings fan, as he is an ancestor of Aragorn's and was responsible for defeating Sauron and taking the One Ring for himself, until it eventually got him killed.
This, of course, means that Elendil is also one of Aragorn's ancestors, which is one of the subtle ways that The Rings of Power is able to tie itself back to the Lord of the Rings story. Elendil's bloodline became the long-lived Dúnedain, of which Aragorn was a part. When Ar-Pharazôn and Sauron began preparing for their attack on the Valar, Amandil was warned of the downfall of Númenor and told Elendil to gather the Faithful (as well as a seedling from the White Tree of Númenor) and sail away from Númenor towards Middle-earth. This is what saved them, and made these people the only survivors of the sinking of Númenor. Upon arriving in Middle-earth, Elendil was welcomed by the Elven-king Gil-galad and founded the kingdom of Arnor in SA 3320.
Elendil's sons went further south and founded their own kingdoms - Anárion establishing the city of Minas Anor in Anórien and Isildur founding Minas Ithil in Ithilien. The two sons ruled this new kingdom that they called Gondor together from Osgiliath. Sauron began to build his power again and attacked Minas Ithil in SA 3429. Isildur came north after that to confer with his father and left Anárion to defend Gondor. Several years later, Elendil and Isildur returned to Gondor with Gil-galad to launch the Last Alliance of Men and Elves against Sauron and his forces.
As the army advanced into Mordor, the Men and Elves were able to lay siege to Barad-dûr, although Anárion was unfortunately killed during this battle. After seven years of siege, Sauron decided to grace the battle with his presence personally. Elendil and Gil-galad were able to overthrow Sauron, but were killed as they did so. Elendil's sword, Narsil, broke when he fell, shattering into pieces that would not be reforged for many years until the sword came into Aragorn's possession. After his father's death, Isildur used the broken sword to slice the One Rings from Sauron's hand and ultimately defeat him (for the time being, at least).
If Episode 3 of The Rings of Power is anything to go by, it seems like Elendil (as well as Isildur) is being set up to be a major player in the events of the story. Clearly, he is quite important to the history of Middle-earth, and it seems likely that they will want to represent this legacy somehow in the show. Besides the eventual connection to Aragorn, Elendil is going to be an interesting character to watch as he rebels against the imminent corruption of Númenor and subsequently influences Middle-earth. Just how much of a central character he's going to be in the show remains to be seen, but he's certainly one to watch.