Plain English Meaning
The lowest spot in the fuel tank, with a small drain on it, where water and dirt settle so you can drain a bit out and check the fuel before you fly.
Full Definition
A low point built into a fuel tank where water and sediment collect, fitted with a drain valve so the pilot can sample and remove contaminants before flight.
Context Anchor
Encountered during the before-flight inspection when the pilot samples fuel from the drain points under the wings or fuselage.
Derivation
Sump' comes from a Middle English word meaning a marsh or low pool — a place where liquid collects at the bottom. In aviation, it keeps that exact sense: the low collection point inside the tank.