What is pressure behind the eyes usually?

Usually benign: The eye is richly innervated and has a dense collection of sensory nerves. These sometimes "fire" giving various sensations of which pressure is one. In most cases this is benign and of no signficance. You should be evaluated by your ophthalmologist if the vision is changing, there is persistent pain, or the eyes seem to bulge outward and become red and irritated.