It's the tempo, beats per minute - Bpm.
As mentioned above, if your tempo is 60 bpm then every beat is one second long, obviously, and a 4/4 bar is four seconds long. It depends on where in the bar you want to put the kicks what tempo you must use to get the kick every second.
To make the time ruler show accurately enough you need to zoom out enough to make the digits fit on the screen.
I think you're overthinking this somehow. It should be really simple. You think you're kicks in seconds, just as the beats per minute tempo setting does.