Instrument stems are the individual audio files that comprise a complete recording. So you might have separate audio files for the guitars, the bass, the drums, and so on. Using stems to transcribe or practice music makes life so much easier.

A fantastic resource for freely-downloadable stems can be found here:

https://linktr.ee/burntstems

From there, you can find many other resources, channels and outposts for stems, so it's a great starting point. Many metal bands have song stems available, in particular Metallica, Megadeth, Pantera, Disturbed, and many others!

If you need help fetching your desired stems, feel free to ask for guidance.