A measure is a value applied to dimensions-–for example, a count of users, time on page, and bounce rate. Explore adds three standard measures to every view: sessions (visits), users (visitors) and views. You can also create and quickly add custom measures to a view.
All measures (both default measures and those you create) appear in the left sidebar under the heading Available Measures. Measures being used in your current view appear under the heading Measures In Use.