Its a alternative version of the normal game, they can be found in the install folder since sins 1.81 and entrenchment 1.041. They have certain features which make modding and especially debugging a dream (well more of a dream). it for example shows all errors in the game, so if had my previus example it would say that it couldn't find the enum value "ApplybuffToSelf". Also if line was written as bufftype instead of buffType it would say that the line number xx in xxxxxxx file named bufftype should have been named buffType (makes upgrading versions a lot easier).
It also allow altering files on the run, so if i change the shield of a unit the change is applied instantly, that way I won't have to constantly reload the mod/game. It can also open a menu which can change a lot of aspects of the current game making testing certain things easier and faster.