From Jonathan Chait's piece, “Forget Impeachment. Mueller's Real Threat to Trump Is in 2020” on the New York Magazine site:

The breadth of Trump's legal exposure exceeds that of any president in American history. It is so vast that it is hard to comprehend. Some, and possibly all, of the following appear to have colluded with Russia on behalf of the Trump campaign: Michael Flynn, Roger Stone, Paul Manafort, Donald Trump Jr., and Michael Cohen. Trump has been doing business with the criminal underworld in Russia and elsewhere for years, the secrets of which may be revealed by Mueller, or by House Democrats obtaining his tax returns. Federal prosecutors are investigating whether he violated campaign-finance laws by directing hush money to various mistresses. The state of New York is investigating the Trump Foundation for alleged misappropriation of funds and the Trump Organization for decades-long tax fraud. He is being sued for violating the Constitution's Emoluments Clause. He is also being sued for fraud.

And this is just the information we know so far, which has come out despite a Congress dedicated to protecting him from investigation, a benefit he will enjoy for only a few more weeks.

As for what the headline means: Chait says it's unlikely Mueller can indict a sitting president; and for impeachment you need 2/3 of the Senate or (currently) 20 Republican Senators. So the real battle will be in 2020 when the voters can do something about it. 

Unfortunately that brings up the scariest line in the piece, According to Chait, a poll from last spring “found that nearly three-fifths of the public is unaware that Mueller has uncovered any crimes at all.”