Click the start point and then the end point to complete the measurement, and the measurement result will be displayed in the lower left corner of the interface, The slope of the measured two points is also displayed.
SketchUp itself also provides a "Tape Measure" tool that can measure length. The difference between the two is that this function measures the projected length of the two points on the horizontal plan, while the tape tool measures the 3D spatial distance between the two points.