The Endurance travels through the wormhole exiting on a system in orbit of Gargantua: it is a very massive, rapidly spinning black hole. It is orbited by the planets Miller and Mann. 

A black hole is a region of spacetime where gravity is so strong that nothing, light included, can escape from it.

Cooper decided to land on Miller's planet, also called the water world. In this planet every hour will be seven years to the outside world due to time dilation. They will spend 3 hours (on the planet) but the rescue mission on the water world fails due to large waves.