According to Wikipedia, a vapour chamber is a flat heat-spreading device and a special form of heat pipe. It is commonly used to move heat away from small hot components and distribute that heat over a larger surface.

Inside a sealed chamber, a small amount of working fluid evaporates at the hot region, then condenses at cooler regions and returns by capillary action.
This evaporation-condensation cycle enables fast, efficient thermal transport across the plate.
Source: Wikipedia - Vapor chamber