|Obstruction of justice. It was one of the articles of impeachment for Clinton, and the first article of those drawn up against Nixon before he resigned.|
Despite what Trump and his loyalists say. Mueller's report specifically does NOT exonerate Trump when it comes to possible obstruction. If you read Volume 2 of the report, that's quite clear - he actually says that because of the DOJ guidance that a sitting President cannot be charged with a crime, he tailored their investigation to avoid the possibility that they would have to charge him with a crime.
That's not an exoneration; Mueller was punting the question to the House, which has no such constraints. The evidence of obstruction - 10 or 11 incidents of it - is in the report. It's up to Congress to decide whether that evidence is compelling enough.
The fact that Barr decided there was no obstruction is immaterial - it wasn't his job to characterize what Mueller said because the facts as presented speak for themselves; Barr has previously said that a President can't obstruct justice, so he had a bias. If you saw his press conference prior to the release of the report, it would have been obvious that he was trying to be the President's defense attorney because he talked about Trump's state of mind that may have led to attempts to obstruct. How did he know Trump's state of mind? He also admitted that he gave Trump's lawyers copies of the redacted version of the report several days before it was provided to Congress and released to the public. Why? Whose interest was he serving - ours or Trump's?
For those reading this who haven't actually read Mueller's report, you can get the official report here:
Volume 1 deals with the investigation of Russian involvement in the 2016 election, and Volume 2 deals with the possible obstruction of that investigation. Volume 2 is definitely a more interesting read.