It is a method of developing film to completion. That is using up all the developer that is in contact with the film. I is a form of compensating development. It is a lazy man's way to develop film because you do not even need to be there after pouring in the developer. It does away with time and temperature worries.

To give you an idea, I normally pour in the developer (Rodinal 1+100 in my case), agitate with three slow inversions and go do something else for an hour. One time I forgot and left it in for 2.5 hours. Ruined? Nope, I could not tell any difference from the one hour time.

Some people, like me, love it. Some hate it. Some, can not leave the tank alone and wind up with very contrasty negatives. Agitating defeats the purpose, and it just becomes insanely long development.