BPM (beats per minute) is the tempo or speed. Select a value between 40 and 320. The default is 120BPM, which is the usual default tempo in music editing software.
If you are not sure what the tempo should be, click on the Tap Tempo button. When the button is green, it is activated. You can either click on the button or press the T key at the tempo of your song. The average tempo of up to 10 taps/clicks is calculated. If you mess up, you can continue tapping the tempo until the displayed value stays about the same. Click the set button to copy the value into the BPM setting.
Move the volume slider to make the metronome louder or quieter.
An Accent note can be set every so many beats. This should match the time signature of the music that you are playing. Most western music is in 4/4 time, so an accent every 4 beats is the default value. The accent can be removed (no accent) or set to any value between 2 and 16.