Plain English Meaning
When a tire skids without rotating on a wet runway, friction can heat the water under it into steam. The steam holds the tire off the ground and the heat melts the rubber, so the airplane slides with almost no braking action.
Full Definition
A form of hydroplaning in which a locked, skidding tire generates enough frictional heat on a wet runway to boil the trapped water beneath it. The resulting steam lifts the tire off the surface while the heat softens and chemically reverts the rubber of the tire's contact patch back to an uncured, gummy state, leaving distinctive white steam-cleaned streaks on the runway and patches of melted rubber on the tire.