Adam Davidson, writing at The New Yorker:

The scenario that, to my mind, makes the most sense of the given facts and requires the fewest fantastical leaps is that, a decade or so ago, Trump, naïve, covetous, and struggling for cash, may have laundered money for a business partner from the former Soviet Union or engaged in some other financial crime. This placed him, unawares, squarely within sistema, where he remained, conducting business with other members of a handful of overlapping Central Asian networks. Had he never sought the Presidency, he may never have had to come to terms with these decisions. But now he is much like everyone else in sistema. He fears there is kompromat out there—maybe a lot of it—but he doesn’t know precisely what it is, who has it, or what might set them off.

Trump and many of his defenders have declared his businesses, including those in the former Soviet Union, to be off-limits to the Mueller investigation. They argue that the special counsel should focus only on the possibility of explicit acts of collusion between the Trump campaign and the Russian government. This neatly avoids the reality of sistema. As Pavlovsky wrote, “Under Putin, sistema has become a method for making deals among businesses, powerful players, and the people. Business has not taken over the state, nor vice versa; the two have merged in a union of total and seamless corruption.”

This explanation is the best I’ve read thus far. Wild theories about The Pee Tape or Trumo being some sort of double-agent, while fascinating and lurid, are too neat.