The Rickety Cart allows you to reset and retire heroes.
This is quite honestly one of the best features in the game.
When you reset a hero, everything you’ve invested into it will be given back to you. The gear it is wearing will be put back into your bags.
It costs 20 Diamonds per hero to reset.
Only common (green) heroes can be retired.
Unless you’re just starting out and don’t have anything better, the only purpose of green heroes is to retire them.