http://www.npr.org/2017/10/22/559086726/the-russia-investigations-interference-impacted-real-life-senators-propose-new-lSo, summary:
Russia basically tried to get people ready to fight each other, dividing the nation best it could.
While I don't think Trump colluded as this seems very "hit both sides" kinda thing, if he did, it would explain why he was so against polls and his "I'll accept the results if I win." statement. What better way to sow discord than to have your unpopular front runner claim the election was rigged, lose, and spend the next year complaining about it as loud as possible, asking for people to rise up against a corrupt system.
Having Trump win probably only made it a little harder to sow discord since the super angry base got what they wanted.
Also: Mitch McConnell must hate his job right now. Having to say "Oh yes, I trust Trump and we're so close." while Trump insults him on twitter.