Stuff is items that will help you grow your help. Stuff can give you extra exp from your crops or more yield from your harvest. It also can reduce the amount of time for you to grow your crops and reduce the time it takes for your crop to die.
Stuff is a great way to improve the amount of money you can make but there is a cap on the benifits it can give.
See: Category:Stuff for a quick list of all types of stuff.