Plain English Meaning
Before putting fuel into an aircraft, you connect it by a wire to the ground so any static electricity safely flows away. This stops a spark from setting fuel vapor on fire while you are fueling.
Full Definition
The procedure of electrically connecting an aircraft to the earth (or to a fuel truck that is itself connected to the earth) before fueling, in order to drain off any static electrical charge that has built up on either the aircraft or the fuel delivery equipment. This prevents a static spark from igniting fuel vapors during the fueling process.
Context Anchor
Used during aircraft refueling, especially before opening fuel caps or starting fuel flow.
Derivation
From 'ground,' meaning the earth itself. In electrical work, connecting something to the earth gives stray electrical charge somewhere harmless to flow. The use is the same idea applied to fueling.