What the expression is is basically to transfer one range of numbers to a scale/new range.
The format is such [expression](x,y,a,b)
X=lowest of values from input i.e. Audio keyframes
Y=highest of values from input
A=new value for lowest
B=new value for highest
I might not know exactly what the individual "parameters" exactly do, but I remember watching a tutorial from I think Andrew Kramer or Aaron Rabinowitz and they used it to animate I want to say audio keyframes to make a bar move up and down. I have gone through all the ones I thought it was in and just can't seem to find it.