There is a certain delay between the program sends Note On event and you tap a bar line. It consists of MIDI Synthesis latency and your reaction time. Calibrating Tapping Delay dialog allows to determine the delay so that the program could compensate for it.
The Calibrating Tapping Delay dialog is brought up by pressing "Calibrate..." button on the Playback tab of General Options dialog.
After pressing Start you will hear the clicks. You need to tap each time you hear the click. The process takes about 20 seconds. You may stop it by pressing Stop. After tapping, press OK button to use the calculated latency with the Tapping tool. Determined latency time is indicated in the Latency field.