To learn a song properly you need its tempo: to set the metronome, to slow it down for practice, to dial in a delay pedal, or to build a drum loop to jam over. Tap tempo is the quickest way to find it. Play the song, tap along with the beat, and the BPM appears after the second tap and gets more exact with every tap after that.
The tempo comes from the average gap between your taps, not just the last two, and a gap far from the rest (a missed or doubled tap) is left out automatically, so a stumble does not spoil the reading. A steadiness meter tells you how even your taps were, and a chart shows every gap against the average.
Songs are often felt at half or double the tempo you tapped, so both are shown as well. Below the reading you get the length of every common note at that tempo, in milliseconds and in hertz: exactly what a delay pedal, an echo plug-in or a tremolo needs to sit in time.
Not sure you tapped right? Check by ear plays a click that starts in step with your last tap, so you can hear at once whether it stays with the music. Then open the tempo in the metronome or the drum machine with one tap. A USB or Bluetooth MIDI pedal works too, which leaves both hands on the guitar.