PokerSnowie Online Manual
The error rate gives a score for how well the session was played. It is independent of the actual outcome and only based on the skill of the player.
For every betting round, the error rate sums up the EV cost of all errors, divided by the number of moves played. So the smaller the error rate the better.
Please note that the sample size needs to be big enough to get a significant error rate. Usually 5K to 10K of imported hands is enough to get a stable error rate.
Very strong high stakes players have an error rate of around 7 in ring games and 5 in heads-up games. Less experienced players and beginners often have an error rate of over 20.Blunders
A blunder is a costly error. The threshold for when an error is considered to be a blunder can be set within the options menu. By default the threshold is set to an EV loss of 2.0 big blinds.
The link 'Browse your blunders' lets you conveniently browse through all big errors, so that you can efficiently learn to avoid your biggest errors.