Ahch-To is a planet where Luke Skywalker took solace after Ben Solo turned into Kylo Ren. It is a watery planet with a few islands. Luke had gone in search of an old Jedi Temple which is what he found. Luke had hoped to be left alone but the Rebel Alliance and now called Resistance had found his location and had sent Rey and Chewbacca to find him and bring back into the fold. The Island briefly appeared at the end of the The Force Awakens but plays a larger part in the follow up The Last Jedi.
Ahch-To is located in the unknown regions which is perfect for someone who doesn't want to be found. On the main island, there is a large tree which inside contains very old Jedi texts which are destroyed when the ghost of Yoda burns the trees because Luke can't bring himself to do it.
The scenes for Ahch-To were filmed in part on a small group of islands off the south western coast of Ireland. The rocky crop of islands were once home to a group of monks who set up there in about the sixth and eighth centuries away from the rest of the populace. The monks abandoned their monastery in about the thirteenth century and moved back to the mainland. Monastic ruins of huts made with stone are all what is left on the island including the steps that Rey walks up. World Heritage Ireland
After being featured in the film, the interest in tourism to the place will have increased. Visiting the island is limited to protect the location from degredation. There are boat trips and a Helipad on the pad on the island for people to visit.
When Rey visits the island, she persuades Luke to train her. Through her training, Rey sees various wildlife on the planet, life growing but also a dark place. The dark place is a hole in the ground which when she dives into it is a underground lake. There, Rey hopes to see her parents but all she sees is her reflection and no answers about her past and her parents.
There are three races indigenous to the planet, of which only one has a level of intelligence. The other two races are just wild-life. Its not right to call them aliens because they are from the planet.
These are giant walrus like creatures that are seen on the cliff side which Luke visits to milk for a drink. There is another creature in the water which we presume to be a Thala-Siren too. The creature is docile and doesn't lash out when milked.
Their proper name is Lanai but they are predominately referred to as Caretakers. The Lanai are the most intelligent indigenous race on the island. Their purpose is to help maintain the location. They clothing resembles that of nuns with their habits. They don't speak English but just make sounds that are understood by Luke Skywalker.
The other wildlife race that we are introduced to are the Porgs. They are small bird like creatures, their looks said to have been influenced by the real Skellig Michael wildlife, Porgs. They are a small cute creatures that make no sounds apart from sqwarking and designed to tug on your emotions.
Unlike Ewoks, they do not fight, instead one Porg sits in the cockpit of the Millennium Falcon with Chewbacca who has grown used to them. Chewbacca was seen to have caught a Porg and cooked it before Porgs arrived and made him feel guilty.
We'll probably see more of the Porgs in the next film as Chewbacca had grown accustomed to them and a number of the creatures were seen aboard the Falcon, not just the one in the cockpit.