A 12-speed mountain bike drivetrain has 12 rear cogs in the cassette, giving you 12 gear ratios to choose from. It's the current industry standard found on all modern mountain bikes from mid-range to high-end, and it uses a single front chainring (called a 1x setup) to keep shifting simple and reduce dropped chains.
The big advantage of 12-speed is the wide gear range—typically 10-52 teeth on the cassette—which means you get both easy climbing gears and fast descending gears without needing multiple chainrings up front. The steps between gears are also tighter, so you can fine-tune your pedaling cadence more easily.
If you're buying a new mountain bike, 12-speed is what you'll find on most models, and it's the best choice for parts availability and future upgrades. It's worth prioritizing if you ride varied terrain and want the simplest, most capable setup.