Rear suspension travel is the maximum distance the rear wheel can move vertically to absorb impacts, measured in millimeters. The right amount depends on the terrain you ride most. For cross-country and smoother trails, 100–120mm keeps the bike efficient and lightweight. Trail riders tackling rougher terrain typically want 120–140mm for a good balance of climbing ability and descending confidence. If you're hitting aggressive enduro courses with big drops and rocks, look for 150–170mm. Downhill park riders need the full 180–210mm.
Keep in mind that raw travel numbers don't tell the whole story. Suspension linkage design and shock quality matter just as much for how a bike actually performs on the trail. Also, rear travel is usually slightly less than front travel on full-suspension bikes, and that's perfectly normal.
Focus on matching your travel to your typical terrain and riding style rather than chasing the biggest number—more travel adds weight and can make the bike feel sluggish on mellow trails.