The solvent heats until it evaporates and travels up to the reflux condenser. Solvent vapor does not escape. It encounters a cooled surface on which to condense. The liquid trickles back to the original boiling solution.
With a constant heat source, the condensate recycles into a solution at a constant temperature. Temperature control at solvent boiling point is a major reason for reflux setup. The evaporation-condensation cycle means that, as long as the heat source is working properly, reflux reactions can go on for a long time--hours, even days.
Reflux heating is done in an open container. Gases do not build up to dangerous pressures. Upon finishing a reflux reaction, slowly cooling the flask is important. Swift cooling may cause the hot glass flask to shatter. Make sure to keep cold surfaces and liquids away from the hot flask for some time after reflux.