The extension icon in the toolbar gives an overview of how uMatrix affected a page:

The red and green areas provide a rough overview of how many resources were blocked (reddish) versus how many resources were not blocked (greenish).^[1]

The badge number tells the number of resources which were blocked/modified. This number will typically match what is reported in uMatrix's logger.

You can turn off the badge number in the Settings pane in uMatrix's dashboard.

_{[1] Keep in mind that "resources blocked" is a generic expression which includes removal/modifcation of response and request headers.}