Your gradebook has 3 modes: Points, Locked, and Percent. Most teachers prefer to use the point mode which allow you to view and enter scores as points.
If you set the mode to "Percent", the cells display students scores as percent of the max possible, but the cells are not editable.
In the "Locked" mode, empty cells are editable but cells with scores are not. This is helps to prevent accidental score changes.