Plain English Meaning
Pulling a small sample of fuel from the low points of the fuel system into a clear cup so you can check that there is no water or dirt in the fuel and that it is the right type.
Full Definition
The preflight procedure of draining a small quantity of fuel from each fuel tank sump and from the fuel strainer (gascolator) into a clear sampler cup, then visually inspecting it for water, sediment, and the correct fuel grade and color before flight.
Context Anchor
You do this during the preflight inspection, usually with a fuel sample cup or tester at each fuel drain point listed for the .