You can use the Tape Measure tool to measure a distance on the map. The distance is measured along a route that you draw with the mouse. You draw the route as connected straight lines.
Click the Tape Measure button to select the tool
A "Tape Measure Length" window pops up. This window shows the length of the current route. Since no route is drawn yet, the length starts at 0 m.
Click the left mouse button where you want to start the route, then click the left mouse button again, where you want the route to bend. The "Tape measure Length" window updates every time you click a new point.
When you click the right mouse button, the last point on the route is removed. This allows you to "backtrack" the route.