The standard deviation is the square root of the variance of a series. It is one of two core measures (along with mean) that help to describe normal distributions.
Consider two NBA teams. Team A always scores 100 points/game. The other team takes 100 independent shots a game (all two-pointers, no free throws) and makes them at an exactly 50% clip. Both teams thus have a mean of 100 points per game, but while Team A has a standard deviation of 0, team B has a standard deviation of 5. If they were facing an opponent that scores exactly 105 points every single game, Team A would win 0% of the time, while Team B would win 32% of the time. In general, underdogs try to increase variance (and thus the standard deviation) while favorites try to decrease variance.