The FBI was investigating certain US officials for being in contact with the Russians while the elections were going on and influencing the elections in some way. Donald Trump fired the FBI director not long after becoming president, while he was under investigation (I don't think he was 'officially' under investigation, just his colleagues etc.). As well as that, apparently he recently released 'top secret' information to the Russians, which poses a security risk for the US, and everything else he's done...
What Congressman Al Green, the guy the article is talking about, is suggesting, is that the president gets 'impeached'; basicly, they get rid of him, along with anyone else that works with him (Vice President etc.).