The FBI's Data Intercept Technology Unit integrates with email and other Internet communication providers to implement wiretaps for accounts they have a court order for. They present the company with a court order, and the company forwards the account's data to the FBI (often via FTP, according to a New York Times article at the time). PRISM is simply a data integration project between the NSA and the FBI for ingesting data for accounts under a Section 702 order (for specific accounts belonging to non-citizens outside the US whose communications are of national security interest).
It's right there on the leaked slide: https://imgur.com/setOJIm