By no means an expert as all knowledge I have of these is via the Universal/IOA board on here. I plan to buy the pass if we decide to vist Universal on Boxing Day.
The provide the equivalent of a "fast pass" for each ride, but with no time restricition.
You can purchase them in advance from www.universalvacations.com by selecting the theme park tickets only option. They cost $20 for peak times and $10 off peak - I believe this is inclusive of tax (not certain) and you don't have to pay postage the only option as international guests is to pick them up at Guest Services at Universal. You can also buy them at the gate, but not sure of cost.
Depending on how many people in your party and how many days you plan to spend at Universal/IOA, it may be worth looking into staying at one of the onsite resorts. These provide a similar benefit at both parks to the Express Tickets, but you can use it more than once for each ride.