First, the headline:Michael Cohen pleads guilty, says he lied about Trump's knowledge of Moscow project
Then the significance:
The Cohen revelations are potentially significant because they appear to show that Trump was engaged in business dealings with Russia in the midst of a campaign in which Moscow interfered to help elect him.
It could also intersect with other information that Mueller knows to create political and legal jeopardy for the President.
Also of significant note is that Cohen explicitly
named Trump, the Trump organization, and Trump "family members" as participants in the scheme to deceive. I think the timing on this is not as has been asserted by others - because Trump is going to the G20 and planned to meet with Putin - but because the Manafort hearing (and Cohen's sentencing) is coming up, and he has gotten answers from Trump, so his case is mature enough to allow the shoes to start dropping. In addition, the charge was about lying to Congress
, which is an unsubtle hint to Congress that they need to reopen the hearings.