A fuel system vapor lock is when the fuel in a vehicle changes from a liquid to a gas state while it is still in the fuel lines. If the fuel is in a gaseous state in the fuel lines, the fuel pump can't push liquid fuel to the engine.
A vapor lock is typically caused by the fuel overheating somewhere in the fuel delivery system. The problem is most common on gasoline powered carbureted engines that have a mechanical fuel pump in the engine compartment. Excessive heat due to weather conditions, engine overheating or high-altitude driving, which reduces the boiling point are common causes. Gasoline with a higher ethanol content is more volatile and can be a contributor as well.
Vehicles affected by vapor lock tend to run roughly, sputter and die. Typically, the engine doesn't start until it has cooled down. Fuel-injected vehicles with an electric fuel pump located in the fuel tank are less susceptible to vapor lock.