The menubar icon display the current status of all active URLs.
In its default configuration it displays a pie-chart where the red part marks URLs with errors, the blue part unchanged URLs, while the green part changed URLs.
You can change the type of icon among a big pie-chart, a smaller one and a vertical bar and you can also configure all 3 colors; see the preferences section for more details.
If you move the mouse pointer over the icon and leave it there, a tooltip will display the number of changed and unchanged URLs and the number of errors.