Start the click and tap along on every beat, using the space bar or the large button. After twenty taps the app shows how early or late you tap on average (the mean asynchrony) and how consistent you are (the standard deviation). The swing section moves the off-beat click later in the beat so you can compare two swing ratios.
What to notice: most people tap slightly ahead of the click. The measurement is approximate, since the browser adds a few milliseconds of audio and input latency.