Definition: The axle is the metal rod that runs through the hangar of a skateboard's trucks.
A lot of people make the mistake of calling the whole truck the "axle", but the skateboard truck is actually made of several parts. But, the axle is the part that holds the wheels, so you are on the right track.
The axle can, after a lot of riding, start to slip to one side. This makes one wheel too loose, and one wheel too tight. Taking the wheels off of your trucks and tapping the tight side can fix it, but I recommend starting to look for a new set of trucks if this happens to you. Some truck companies add a bend to their axles in the middle (hidden by the hangar), to that this won't happen.
Examples:
"Kentaro uses spacers on his axles - little rings that add a little space between the wheels and the hangers."
