"Stuff" can mean almost any physical items or material.
It's similar to the word "things", but "things" are countable and "stuff" is not. So "stuff" is better to use when you're talking about a group of things that are different from each other - different sizes, shapes, etc.
"Stuff" is especially used to refer to things that you buy or own:
But "stuff" can also mean "topics" or "ideas", like in this example:
We have a lot of stuff to talk about.