Plain English Meaning
A gas cap with a tiny air hole in it. As fuel leaves the tank, air comes in through the cap so the tank doesn't suck itself flat and starve the engine.
Full Definition
Fuel tank caps that contain a small built-in air passage allowing outside air to enter the fuel tank as fuel is drawn out, keeping the tank's internal pressure equal to outside air pressure. This prevents a vacuum from forming inside the tank, which would otherwise restrict or stop fuel flow to the engine.
Context Anchor
You encounter vented fuel caps during preflight inspection, especially when checking that the fuel caps are secure and that any vent openings are clear.
Derivation
Vented' comes from the Latin 'ventus' meaning 'wind' — a vent is simply an opening that lets air move through. A vented cap is a cap with a deliberate air passage built in.