It's really wonky, and IMO not at all useful, but ideally, if there is 1 footprint then you are almost on top of the pokemon, 2 it's further away, approx. a 1/2 block, and 3 is ridiculously far...try walking in a direction and you'll see the footsteps(hopefully) go from 3 to 2, that means you are getting closer, if it gets back to three then keep adjusting your route to hopefully get closer...in my experience they are too hard to find and it takes to long to get there, they usually disappear even as I am getting down from 3 to 1 footstep.
Also, in the 'nearby pokemon' menu on the map screen, you can select 1 of the pokemon and it will be the only one in the little guide bar, which should help you to see if the footsteps are getting further or closer