From their website:

"Make the most of your time at Universal Orlando Resort™ with Early Park Admission*. On-site resort hotel guests and other select groups can access one theme park and [certain] attractions an hour before the scheduled opening."

There's also something called the Express Pass -- basically, a "front of the line" pass for specific rides (including many of the Harry Potter rides). You can buy either one-time (e.g., you can go to the front of the line for certain rides one time per ride) or unlimited (front of the line for certain rides, as many times as you'd like). $40 vs $100. If you stay at some of the higher-end on-site places, the unlimited pass looks to be free with your stay. If not, definitely consider purchasing one of them if you're going during prime days.

One tip if you get the non-unlimited pass but stay on resort -- do Harry Potter first when it opens just with the regular lines. Then when the park opens to everyone else, leave HP World and go on some of the other rides without using your pass. The lines at HP are going to be fairly short during that early access hour, and then when the park overall opens, everyone runs to HP, packing that solid but leaving the rest of the park with shorter rides for about an hour, until everything seems to settle into normalcy for the park.

Try googling through the blogs to see when good and bad days are for crowds, if you're somewhat flexible. I know one blog for Disneyland gave general suggestions for when their peak crowd days are, so I'd assume Universal Orlando does the same.