270 electoral college votes are required to win the election, which is why the graphs changes color. If no candidate wins at least 270, Congress chooses, I believe. 538 is currently giving the chances of no candidate hitting 270 at 1.4%[1].

The y-axis is the probability of that particular outcome (or range of outcomes?) occurring.

[1] http://projects.fivethirtyeight.com/2016-election-forecast/?...