Let's say the current is 0. Then the output will be Vout=Vref.
Now let's say that 60A of current is flowing and you have the HTFS 200-P so that Ipn is 200. Then the output will be Vout = Vref + 1.25*60/200 or about Vref+0.375V.
If -60A of current is flowing (other direction) then Vout = Vref - 1.25*60/200 or about Vref-0.375V.
Note that even when current is 0 then Vout=Vref+/-0.025V so there is some uncertainty in the measurement.
The Vref pin is useful to be able to measure both positive and negative currents when you have a single-supply (0V-5V) system rather than a split-supply (-5V - +5V) system. If you set Vref to 2.5V then you can consider voltages between 2.5V and 5V to be "positive current" and voltages between 2.5V and 0V as "negative current". If you know current is only ever going to be flowing one way I suppose you could set Vref=0.
The Rugged Motor Driver
: two H-bridges, more power than an L298, fully protected