Spoilers ahead for Star Wars: The Last Jedi. In Star Wars: The Force Awakens, it was made clear that Luke Skywalker, wherever he was, did not want to be found — and not just because his disciple-turned-mass murderer wanted to kill him. At the end of TFA it looked like all Luke wanted to do was hide from his past on an island, but actually what Luke Skywalker's island is in The Last Jedi is much more than a tropical getaway.
Before Rey tracks Luke down in The Force Awakens, the only thing she knows about Luke's whereabouts is what Han Solo tells her: "The people who knew him the best think he went looking for the first Jedi temple." And in The Last Jedi, it looks like he found it. Unnamed in TFA, The Last Jedi reveals Luke Skywalker's island to be Ahch-To, home to the first Jedi temple. A water-based planet, Ahch-To is a sacred place, not just to Luke, but to all who believe in the Force. Fans will recall that in Rogue One, the Force is presented as a kind of religion — not just a mystical energy or power for the Jedi, but a faith for everyone. In The Last Jedi, Luke calls it the "Jedi religion" while explaining the island's significance to Rey.
Not only is Ahch-To the site of an ancient Jedi temple, it's the first Jedi temple. Luke has taken refuge in the one place that holds the history of the Jedi. As the first Jedi temple, Ahch-To is where ancient texts detailing the history of the Jedi are housed. Dating back centuries, the texts describe the first Jedi and their connection to the Force. It's in studying these texts, Luke tells Rey, that he has come to believe that the Jedi must die. Luke believes that the Jedi should die on this island — it is where he has come to die, hoping that the Jedi will go with him. But when Rey finds him, it's clear that the island is more than just the site of the first and last Jedi temple or the place where the Jedi end.
Luke's island in The Last Jedi appears to be a vergence in the Force, something Nerdist described as a place (or person) that is exceptionally strong with the Force. As such, Ahch-To plays a big part in the balance between Light and Dark. This is made clear through Rey's training with Luke, when during her first Jedi-guided meditation, she sees the island as representative of the Force itself. The light is above ground — the sky, the nature, the beauty of it all — and the dark is somewhere below — a mysterious, menacing black pit located near the ocean. The island is literally the place where Dark and Light meet, and where both sides call out to Rey.
As with everything else involving the Force, Ahch-To remains somewhat of a mystery at the end of The Last Jedi. (One major mystery: what exactly was that mirror Rey found below ground, in the Dark?) And it's unclear whether or not fans will ever really know everything about the mystical place. One thing we do know, however, is how Luke Skywalker got to Ahch-To, home to the Porgs and the nun-like Caretakers, in the first place. This question is answered in the video game Star Wars Battlefront II. The game, according to Den of Geek, confirms what fans have long suspected: that Luke went to live on Ahch-To after Ben Solo (now known as Kylo Ren) turned away from the Light and pledged allegiance to Snoke and the First Order. In the game, Luke doesn't find Ahch-To all on his own. Instead, he follows a calling to the planet of Pillio, where Emperor Palpatine had a vault. Luke steals a compass from the vault, an item Den of Geek proposes led him to Ahch-To.
At the end of The Last Jedi, Luke is dead and the ancient Jedi tree where the texts were held is burned to the ground. The Jedi of the past have died to give way to the Jedi of the future, which means that even as the Jedi live on, it seems unlikely Episode IX will see a return to Ahch-To. But, hey, once Luke's island, always Luke's island. Fans of Ahch-To will see it again, one way or another.