The informant’s work
According to press reports, the FBI had been interested in Page for several years once it found suspected Russian agents talking about recruiting him. Papadopoulos became a target after the first dump of Wikileaks emails that undercut the Clinton campaign. An Australian diplomat told the FBI that Papadopoulos had known about the emails months before they came out. (Papadopoulos later pled guilty as part of the special counsel’s Russia investigation.)
The informant, according to reports, would create opportunities to talk to the Trump team members and report what he learned to the FBI. If he did more than that, it has not been reported.