Plain English Meaning
A small filter at a low point in the fuel system that catches water and dirt so they don't reach the engine. The pilot opens a drain on it during preflight to check for and remove any contamination.
Full Definition
A filtering device installed at the lowest point of the fuel system that traps water, dirt, and other contaminants before fuel reaches the engine. It includes a drain valve that allows the pilot to sample the trapped fuel and contaminants during preflight inspection.
Context Anchor
Seen during preflight fuel checks and in aircraft fuel system descriptions, especially where the pilot drains fuel samples to look for water or debris.
Derivation
Strainer' comes from the Latin 'stringere,' meaning to draw tight or squeeze. In everyday use, a strainer separates from liquids -- the same idea applies here, with the added job of catching water that has settled of the fuel.