Metronomes & Tuners
A metronome is a device used in practice that produces a stable pulse (or beat) to help musicians play rhythms correctly. The pulses/beats are measured in beats-per-minute (BPM).
A tuner is a tool that senses and shows the pitch of notes played on an instrument. "Pitch" is the highness or lowness of a note, which is measured in Hertz (Hz). Tuners indicate whether a pitch is above, below, or equivalent to the wanted pitch.