There are three measure tools: length, area and angle. With them, you can measure distances, areas and angles on the map canvas. The tools provide results in the units set in <label>Settings > Options > Map tools</label>.
To measure lengths, select the tool and click along the path you want to measure. The length of each segment is displayed as is the total length for the path.
To measure angles, select the tool and click on three points to create an angle between these points. The second point selected is the vertex of the angle. The angle is dynamically displayed once you clicked the second point.