Dropper post travel is how far your seatpost can drop the saddle out of your way on descents — typically ranging from 60mm to 230mm. More travel is almost always better for technical riding because it gives you more room to shift your weight around the bike, so the goal is to get the maximum travel your frame can accommodate.
The right amount depends mainly on your frame size and seat tube length. As a general guide: small and medium frames usually fit 100–125mm of travel, medium to large frames handle 150–170mm, and large to extra-large frames can fit 175–210mm.
Before buying, check two things: that the post can be fully inserted into your frame (watch for water bottle mounts, suspension pivots, or bends in the seat tube that could block it), and that the total extended height still allows full leg extension for efficient pedaling. The overall stack height matters, not just the travel number itself.