Stems are audio files which consist of individual instrument tracks, isolated from the rest of the band, and usually (but not always) sourced from the official recordings. For example, you might find a set of stems for a metal song, where you have one audio file for the guitars, one for the bass, one for the drums and one for the vocals, all separate and 'clean', and all from the actual recordings used on the album track. Using these files, you will have a significantly easier time transcribing/tabbing music, or indeed just learning how to play certain parts.
They're normally files shared amongst music remixers and DJs, and often come straight from the artists themselves (sometimes as part of a mixing competition, or simply sold as stem packs). However, one great and unexpected source of stems for metal and rock is video games, namely games like Guitar Hero. The way these games work required their developers to have access to separated instrument stems, and some industrious tech wizards found a way to rip them from these games to be shared online. This is where 99% of metal and rock stems come from, in fact.
Many stem packs (including some from the aforementioned video games) are actually cover versions, or sometimes a rerecorded version by the original artists, so it's important to be cognisant of this when searching for stems for the purposes of transcribing them.
In addition to finding stem files online, you can also use AI tools to generate stems from music files, although with extremely sketchy results for the most part. If you just need drums or a clean vocal track, these tools are really good. But if you want distorted rhythm guitars separated from a distorted, wah pedal-heavy guitar solo, they're next to useless.
You don't need to depend on online AI tools, though. Thankfully, there's a free/open source tool that runs on your PC. It's called StemRoller. It will generate stems that are of equal quality to any of the online browser-based tools, and it does so without nagging you to register accounts or pay for credits. For drums, it's very good. For everything else, your mileage will absolutely vary! It also has a handy in-built YouTube search tool that lets you generate stems directly from YouTube videos.
Some good starting points for finding stems online can be found below:
If anyone is unsure about how to get a hold of their desired stems, feel free to message me directly and I'll help you out.