It can also be used to mean “eat a large amount.”

Some foods can be stuffed when they are filled with another kind of food.

As an noun

In this example, the speaker is talking about the items that have been left around the room.

This time, stuff refers to the guests’ jackets, bags, etc. 

Stuff is a very convenient way to refer to things as a group when the individual items are unknown or not important.

We don’t know exactly what's in the bag, but it's not important for us to know; the main thing here is the weight of the bag.

Here, as well, the meaning of stuff isn’t clear. If you want to know specifically, you can simply ask.

A

What kind of stuff did you buy?

B

Shoes and lots of cosmetics.

Figurative stuff

When using stuff figuratively, the rules aren’t so strict about what is singular or plural.

Similar to what we’ve mentioned earlier, when what you are talking about is unknown or not important, you can call it "stuff."

Here, it refers to activities. Perhaps the person needs to visit the bank, do their laundry or study for a test. Again, if the other person wants to know about the specific activities, they can ask.

In this example, it means various styles of martial arts.

This time it refers to something someone said.

In this example, it is a particular kind of music.

In this example, “stuff” refers to someone’s thoughts. 

Etcetera

Another very common usage in casual conversations is the expression "~ and stuff." It's another way to say "etc." or "and other things like that."