Definition: When a nitro engine has been running too lean, the extra heat gets into the carburetor and boils away that fuel and shuts down the engine from lack of fuel. This condition is referred to as vapor lock.
Vapor lock usually occurs when you run your RC (especially a poorly tuned RC) for 15 minutes or longer without rest. Tuning the engine so it doesn't run too hot as well as inspecting your fuel line for leaks (that can let in air and lean out the air/fuel mixture too much) are ways to circumvent vapor lock.