The group of muscles that cause frown lines between the eyebrows are known as the Glabella region and they are depressor muscles. They cause the frowning motion, pulling the Corrugator muscles (underneath the eyebrows) and Procerus muscle (between the eyebrows) downwards.
Over time your skin can become marked with consistent contraction.
People having this region treated often often complain of looking annoyed even when they are not. They feel that people think they are angry or scowling, even when they are at rest. Regular treatment of these muscles with botox, every 3-4 months can keep these frown lines at bay.