To tell the truth, not much. Michael McDonald of the US Elections Project believes there is real signal there about ultimate turnout in the election, which he believes will be quite high. Fair enough.
I am less enthusiastic about various analyses that infer a particularly good Democratic election from the heavy Democratic lean of a lot of the early vote. It may indeed by a particularly good Democratic election--in fact, I think that's quite likely--but the early vote patterns aren't really a clear indicator of this. Read Sean Trende's piece on RealClearPolitics for the basic reasons why you should be skeptical.
"It’s that most wonderful time of year. After endless speculation, analysis, and hedging, we are tantalizingly close to having actual new election data to work with. Given this, it is a natural temptation for analysts to gravitate toward the one piece of hard data that we have in our possession: early voting numbers. Take, for example, this piece from Politico, claiming that the Democrats’ massive lead in early voting is a “warning flare.”
I have given my response on this matter before: Don’t try to divine election results from early voting returns. I made this point in 2016, when people were making the exact same arguments about Democrats’ chances off of similar data."