The Foreign Intelligence Surveillance Act of 1978 is a United States federal law which prescribes procedures for the physical and electronic surveillance and collection of "foreign intelligence information" between "foreign powers" and "agents of foreign powers" suspected of espionage or terrorism. The Act created the Foreign Intelligence Surveillance Court (FISC) to oversee requests for surveillance warrants by federal law enforcement and intelligence agencies. It has been repeatedly amended since the September 11 attacks.
Approval of a FISA application requires the court find probable cause that the target of the surveillance be a "foreign power" or an "agent of a foreign power", and that the places at which surveillance is requested is used or will be used by that foreign power or its agent. In addition, the court must find that the proposed surveillance meet certain "minimization requirements" for information pertaining to U.S. persons.
No, I wasn't joking... A federal court judge (very likely) found probable cause that Paul Manafort (Trump's campaign manager during the RNC) was acting as an "agent of a foreign power"... in this case, it just happens to be the same foreign power with which the Trump campaign is being investigated for colluding with during the 2016 Presidential campaign. What a coincidence.