Maximum tire clearance is the widest tire a gravel bike's frame and fork can fit, measured in millimeters. It's one of the most important specs because it determines how rough the terrain can get and how comfortable your ride feels. Gravel bikes typically range from about 32mm up to 57mm of clearance.
For mostly road riding with occasional gravel stretches, 35–40mm is plenty. If you want a true all-rounder for mixed surfaces, 40–45mm is the sweet spot. Riders tackling rough singletrack or planning loaded bikepacking trips should look for 45mm or more. More clearance is almost always better because it keeps your options open if you want wider tires later.
One thing to watch: a tire's actual width on your rim often runs 2–4mm wider than its labeled size. So if a bike has 42mm max clearance, a nominal 38mm tire might actually be too snug. Always leave a little breathing room.