There are a few different ways of using the word "stuff." If it’s a noun, it means things or activities and it can be used for just about anything; if it’s a verb or an adjective, it means to fill or put things into something.
In the examples below, the word "stuff" is a noncount noun:
things
Is this your stuff?
Whose stuff is this?
I have a lot of stuff in my house.
How much stuff do you have in your car?
Do you have a lot of stuff in your pocket?
This stuff is bad for you. You shouldn’t drink it.
activities
We had a lot of stuff that we needed to do after school.
Sheila likes to do stuff with her friends.
What kind of stuff does she like to do?
The stuff that they’re doing in class is very interesting.
In the next set of examples, the word "stuff" is a verb and it means to fill:
simple
past
past participle
stuff
stuffed
stuffed
The children stuffed their mouths with candy.
We stuffed our suitcases with our things and left the hotel.
Many people stuff a turkey with stuffing (bread cubes, celery, spices, nuts) before it goes into the oven.
Those who stuff themselves with junk food get no nutritional value from the food.
The word "stuff" can also be used as an adjective:
stuffed animals