The rest field is basically a field storing the initial positions. For a fluid sim, we can use a rest field for noise lookups in the shader to add apparent detail to your sims at render time. The rest field is advected along with the fluid and therefore the noise can look like it's moving with the fluid when you render it. Further, in houdini the default is to have 'dual rest fields' so that your noise doesn't get too streaky. You have two positions for looking up noise values that are reset every x number of frames, they offset in time from one another so that when one rest field is reset, the other is in full swing, then vice versa. The noise crossfades into the other so that when you add noise to your sim, you get a smoother, and not as streaky effect.
Edit: not much in the way of tutorials for this I think. I have a file set up here that breaks up the rest field setup on it's own and I apply it to a flip sim instead. It may help you get the idea by seeing it on a mesh instead of a pyro sim.. http://fx-td.com/?p=329