Crait is the location for the final battle between The Rebel Alliance/Resistance and The First Order at the end of Star Wars VIII - The Last Jedi. The First Order is now under the control of Kylo Ren after he killed Supreme Leader Snoke so that he didn't have to kill Rey. Kylo Ren wants Rey to be beside him as he leds the First Order to victory.
The surviving remnants of the Resistance had escape D'Qar aboard a few frigates but they were forced to flee the remaining frigate in escape pods and make it to Crait. Admiral Amilyn Holdo gave the Resistance time to get away by destroying Supreme Leader Snoke's supermassive star destroyer.
It is the first time the planet gets a mention in the films but in the storyline, it is not the first time that the Rebels have been to the planet. The Rebels had set up a base which they then left whilst fighting The Galactic Empire. The story of them being on the planet is to be convered in a comic 'Storms of Crait' that will see Luke Skywalker and Princess Leia on the planet for the first time.
The Galactic Empire / First Order thought it not worthwhile to either destroy the base or have a permanent battalion in the Base to prevent it from ever being used again by the Rebels. The First Order unleash all their weaponry including both old and new versions of the AT-AT Walker.
The Rebel Alliance had a base on planet carved into the mountains. The entrance is similar to the entrance that you see when the Rebel Alliance are on Yavin IV.
The only indigenous life forms that we see on the planet are the animatronic / computer generated Vulptex or Crystal Fox. The Vulptices operate in packs like wolves and when the defence shield is coming down, a pack of the wolves run into the base. When the Resistance need to find a way out of the base, Poe Dameron suggest following the foxes that are in the base. The vulptices lead them to a small hole, too small for the Resistance to clamber through. On the other side of the collapsed entrance is Rey who uses the force to move the boulders and get everyone out.
In the background of the picture is the large entrance to the once used Rebel Alliance base. The large metal door comes down but it will prove ineffective against the might of the First Order's battering laser ram.
If you fancy visiting the planet, you won't need to take a trip on the Millennium Falcon, just a drive and a plane ride (if necessary) to the Bonneville Salt Flats, Utah in U.S.A. The Bonneville Salt Flats are located in north west area of the state. Don't do as one of the characters does and touch the ground and then put it in his mouth to say salt.