If you're not into the business world, an IPO is an Initial Public Offering for stock. Zynga, which has been a privately run company until now, is going public. As in $1 Billion (with a B) IPO.
If you're not into Facebook, then you've probably not seen much of Zynga. They are the Farmville, Cityville, Cafe World, Mafia World... (etc.) people. I do the first three and not very well. I was trying to make my city in Cityville to look like Walt's version of Epcot, but it requires too much land. No wonder Walt was so secretive about buying up all that property in Florida.
$1 Billion is a lot of money, however for the amount of revenue it brings in on it's games, it's a fair estimate of what it can raise.
Congrats to Zynga and good luck with the IPO!