https://www.mcclatchydc.com/news/investigations/article219016820.html
A report from McClatchy DC (an outlet that has gotten some big scoops in the past) indicates that Robert Mueller has evidence that Trump's personal lawyer Michael Cohen was in Prague to meet with Russian officials in August 2016. The evidence includes cell tower records of Cohen's phone and an intercepted conversation between Russians. McClatchy claims they have verified the story with four independent sources.
This is significant because the infamous Steele Dossier alleged that this meeting took place and that it was essentially ground zero of the Trump/Russia collusion scheme. Cohen allegedly discussed how to channel money to Russian hackers targeting the Clinton campaign, and how to conceal the Trump campaign's involvement. Cohen has repeatedly denied that he has ever been to Prague.
Obviously this is a huge deal if true. It's the sort of evidence that would show up in articles of impeachment, although I'm sure Congress will wait until they get the full report from Mueller before they commit to anything publicly.