Stuff is a very confusing thing as it can be almost anything.  It can be an idea, or it can be a physical thing or a process of doing things.  Physical things are made of stuff are usually called "objects".

All things are made of other things but ultimately they're all just made of stuff.

Stuff is made from things like atoms and molecules and particles and sub-atomic particles which all stick together somehow with forces.

Some examples of things made out of stuff:

- the Solar System

- the Sun

- the Earth

- land

- water

- humans

- everything else

See also:

- "Doing stuff"

- "Thinking about stuff"

- "Good stuff"