A circle graph is a graph in the form of a circle that's divided into pie pieces with each pie piece representing a part of the whole data set. In other words, each pie piece represents a percentage of the whole pie. Another word for a circle graph is a pie chart.
A circle graph is used to display data and it's used to recognize parts of a data set. It's useful to easily interpret what's going on in the scenario.
An example of a circle graph is provided in the image attached.