Vapor Lock
Vapor lock happens when the fuel in your fuel lines vaporizes, turning from a liquid to a gas. This is most likely to happen to the fuel injection lines in a fuel-injected engine. On hot days after the engine has been shut down, the fuel in the lines can vaporize.
When your fuel lines are full of vaporized 100LL, your fuel pump can't get liquid fuel to the injector nozzles, which makes starting your aircraft frustrating, difficult, or impossible.
Vapor lock can be remedied with a boost pump, if available.