What does 'tire clearance' mean? What does 'tire clearance' mean? Can you please explain with an illustration? Does it mean the distance between the tire and the fender?
Basically its the gap between the tyre and any non-wheel part of the bike frame.

Static clearance is almost exactly the same as the clearances you have while riding, as long as the wheel axle bearings aren't sloppy and the wheel doesn't move laterally.



The above picture is faked up but shows the approximate differences between a "normal road tyre" and a 40mm cyclocross tyre and a 2.1" MTB tyre.  You can see that any smaller tyre will fit, but bigger tyres need more space.

Here's someone with plenty of clearance, and they could easily go up a tyre size or two.


Here's a bike with almost no clearance.


That last bike might not even take the same size tyre from another brand, because tyres vary subtly, and that's not a lot of tolerance!



NOTE just to be clear - tyre clearance matters anywhere that the rotating tyre comes close to your bike frame.  So that's


Chainstays
Seat stays
Brake Bridge
Rim calipers if you have them.
Seat post (pretty unlikely to cause problems)
Mudguards/fenders
Rack supports (unlikely to be a prob)
Trailer mounts
Kickstands &lt;-- ESPECIALLY loose ones!