Front suspension travel is how far the fork can compress to absorb impacts, measured in millimeters. More travel means the bike can handle bigger hits and rougher terrain, but it also adds weight and can make the bike feel sluggish on smoother trails.
The right amount depends on where and how you ride. Cross-country riders typically want 100–120mm for efficiency on climbs and rolling terrain. Trail riders usually go with 130–150mm for a balance of climbing ability and descending confidence. Enduro riders need 150–170mm to handle aggressive descents and big drops. Downhill bikes run 180–200mm for maximum impact absorption on the gnarliest tracks.
A common mistake is assuming more travel is always better. If you mostly ride flowy singletrack or fire roads, a 160mm fork will feel heavy and unresponsive compared to a 120mm fork. Also, never swap in a fork with significantly more travel than your frame was designed for—it changes the geometry and can make handling unpredictable.