Can someone explain to me what STEMS are???

^^^ wouldn't those just be....... tracks?

Stems are usually stereo files of specific instruments. i.e., a drum stem or guitar stem. So instead of having 16 drum tracks, you just get a stereo file that has all the drums. Some people frequently print mix stems instead of the whole song, so they can later alter the volume of just the vocals or guitars but keep everything else the same.

You probably run into that term here where people give out songs to mix. They will come with a drum STEM (all the drums mixed to a stereo file) and separate guitar tracks you can pan or mix however.
 
The definition of stems is whatever you make it.

People commonly refer to them as full, edited stereo tracks for individual instruments, like mva801 said.
I know that a lot of "electronic" music producers refer to stems in that way because that's how they do remixes, as far as I know.

And on here, people often refer to stems as raw tracks for mixing. I guess it can be either, or.