A "pitch raise" is a quick, rough tune. This is requried before the final tune if a piano is way out of tune. "Way out of tune" is relative and depends a lot on the precision of the final tune required and the abilities of the instrument. As a super rough rule of thumb, if your piano hasn't been tuned in more than a year, it probably needs a pitch raise.

Why is a pitch raise necessary? When a piano is left without a tune for too long, the entire instrument drops in pitch as all of the strings (wires, actually) lose tension. Piano strings exert between 18-30 tons of tension on the pinblock and plate. As you can imagine, if you let this tension relax and then suddenly tighten it back up, this causes all of the other parts of the piano to flex. This flexing affects the tension of all the other pins, creating a moving target. As the pins are tightened, the tension (and thus the tuning) of the other strings is affected.

Therefore you have to get a piano at roughly the correct tension before you can make the very fine adjustments to the pins required for a great tune job. Trust me, we wouldn't do it if we didn't have to! We'd much rather spend 45 minutes regulating and voicing.

Here's some info from the Piano Technician's Guild on pitch raises.

What's this regulation you speak of, you might wonder. Want to make me happy? Ask me about regulating your piano.

Here's some info from the Piano Technician's Guild on regulation.