Plain English Meaning
A small valve under the airplane that lets you take a fuel sample from the strainer before flight, so you can check that the fuel is clean and the right type.
Full Definition
A spring-loaded valve at the low point of the fuel strainer that allows the pilot to draw a small sample of fuel from the system during preflight. It is used to check for water, sediment, and the correct fuel grade by color, and to confirm that fuel is reaching the strainer.
Context Anchor
You encounter it during the preflight inspection of the , usually when sampling fuel before the first flight of the day or after .