Now I Get It: What is Trump’s power to pardon?

This week, the first charges were filed in special counsel Robert Mueller’s investigation into possible links between Russia and Donald Trump’s presidential campaign. Three former campaign advisers, Paul Manafort, Rick Gates and George Papadopoulos, each face prison and fines if convicted. In July, Trump tweeted that “the U.S. president has the complete power to pardon.” But what does that power amount to?