"Stuff" is like "things". It's really just a lazy word that is used in English to refer to many things that are grouped together. For example, if I was about to leave the house and my hands were full, I might say to my husband "Can you please grab my stuff?" (keys, cellphone, water bottle, purse)

As Whitney says, it's a lazy word for 'things' in general. For example, if you're busy, you might say 'I've got a lot of stuff to do.' Or if you went shopping and bought many things, you'd say 'I've bought a lot of stuff'. Just a point about the grammar of the word. Note that it is UNCOUNTABLE. So you can't say *a stuff, and you can't say *stuffs. Just 'stuff' in general.