It's a latin expression that means "simply, it's that". Used in order to emphasize that the things are in this way and there is nothing that needs a clarification.
A nice extract:
The Windows Vista product lineup consists of six versions, two for businesses, three for consumers, and one for emerging markets: Windows Vista Business, Windows Vista Enterprise, Windows Vista Home Basic, Windows Vista Home Premium, Windows Vista Ultimate and Windows Vista Starter. The number of offerings is the same as the number of offerings currently available for Windows® XP. More important, the lineup is designed to deliver clear value to a broad range of customers, each product tailored to meet specific needs of various segments of customers — home PC users, small and medium-sized businesses and the largest enterprises — and is aimed at bringing 64-bit, Media Center and Tablet PC functionality into the mainstream.