One of the issues Attorney General Bill Barr is investigating concerns a briefing in 2016. There was a counterintelligence defensive briefing of the Trump team in mid-august 2016 to warn of national security threats. Peter Strzok organized the briefing.
The investigation into collusion was opened in July 2016 by Peter Strzok. Two campaign aides, General Michael Flynn and George Papadopoulos, were already under investigation at the time.
Peter Strzok did NOT warn the President of the threat posed by the aides.
This is important because days before the briefing, Strzok and Lisa Page exchanged controversial emails. Page wanted assurances Trump would never become President and Strzok answered that he would NOT become President.