What is obstruction of justice?

The Robert F. Kennedy Department of Justice Building in Washington, DC. Photo credit: Coolcaesar/CC BY-SA 3.0, via Wikimedia Commons

In his letter to Congress, U.S. Attorney General Bill Barr wrote that Special Counsel Robert Mueller’s report showed no Russian collusion during the 2016 presidential campaign. President Trump says the investigation “totally exonerated” him. But the Mueller report was more ambiguous on the question of whether Trump obstructed justice.