The way humor's usually used in horror, it's as a pressure-release valve; without it, the drama would escalate out of all control almost immediately.

This quote means that when humor is used in a horror movie, it serves the purpose of giving the audience a break from the scary part of the film. This is important because it prevents the scary parts from getting too extreme and making the audience feel overwhelmed. Without the humorous parts, the film might become too intense and out of control.