The predicate was when a Trump campaign advisor told an Australian diplomat that Russia had politically damaging material on Clinton.  He was referring to the hacked DNC emails that were released on the same day as the 'grab them by the pussy' tapes. 

The investigation revealed the previously unknown details of how Russia influenced the campaign to benefit Trump and the multiple connections between members of the Trump campaign (many now convicted felons) and Russia.  As I've said before, you really should read it if you want to have an informed opinion.  In addition to the executive summary, Volume 1 -  Section 4 is a good place to start: https://www.justice.gov/storage/report.pdf