A metronome is a practice tool that produces a steady pulse (or beat) to help musicians play rhythms with accuracy. Beats are measured in beats per minute (BPM).
Most metronomes can operate between 35 and 250 BPM.
Musicians commonly use a metronome to maintain a consistent tempo during practice and to master challenging sections of a musical piece.
In the practice of Pranayama, the metronome helps maintain rhythmic breathing, ensuring a steady ratio among the four phases of breath:
Once you learn to keep a steady rhythm, Pranayama can be practiced without a metronome, with mental counting instead.
Many metronome applications are available online.
The metronome I commonly use is:
https://www.metronomeonline.com/
Set the metronome to 60 BPM, so that each beat corresponds to one second.