This is an old method to add flavor and reduce acidity in tomato dishes.  Illustrated below are a few of the fundamentals. 

Here is the start, cooking the moisture out on a medium fire.

Here most of the moisture is gone.

This is what the sauce looks like when it's scorching. Notice how the edges look like they're curled. The sauce actually begins to slightly fry. Burning will occur if left unattended, stir it often and turn the fire to med-low. The sauce will darken a little as it is scorched, compare the previous picture and the one below. Fifteen to twenty minutes is all it needs to be scorched. Like a roux, if you burn it, it's trash.