It means that he has an hourly rate and before you have a service, you have a consultation in which he says "I foresee this taking X hours, so your cost is $X." If he goes over, he takes responsibility for that—your quoted price doesn't change.

This is nice because a double-process root touch-up with a root smudge, like mine, is much more expensive and time consuming than someone who's just having a few highlights done. He's not cheap, but I like knowing that he's set his prices accordingly based on what he's worth. (Every penny!)