Win rate is a generalization of winning probability for multiplayer games.
A win in a 2 player game is worth 2 points, a 3 player game is worth 3, and so on. Someone who plays only 2 player games and wins every time will have a winrate of 2; a 2-player gamer who loses every game has a winrate of 0; a 2-player gamer who wins exactly as often as expected (half wins, half losses) has a winrate of 1.
The win rate of an event is the average number of points for that event. So an event that is uncorrelated with winning will have a win rate of 1.0, and events correlated with winning have a win rate of > 1.0, and losing with < 1.0.
The error bars represent a 95% confidence interval in the measure for a particular event.
The smaller the bars, the more times a particular event was observed, and hence the less likely the true value is signficantly away from the center point due to randomness.
The error bars represent ±2 standard deviations. They encode the true mean for any particular point with a probability of about 95%.
Note that these statistics are merely an average over many games. They do not necessarily say much about any particular game. So even though one card's early winning rate may be higher than a different card's winning rate on the same turn, it does not follow that the more winning card is neccesarily the best choice for any particular game.
The apparent weirdness with the Curse graph is due to the fact that Curses are rarely intentionally gained. The only exceptions are when players are ending the game (almost always in a winning position) by running out the Curse pile, or are gaining Curses as a side effect of Embargo tokens. When people buy things despite being Embargoed, they are often buying a very valuable card.