Definition of variance
Variance in poker can be understood in hourly rates. A winning player who would earn $50/hour on average, may in a 4 hour session, have won $100 in the first hour, lost $50 in the next hour, won $200 in the next hour and lost $50 in the 4th hour. This is just one possibility. He could very well have made $100 every hour or lost $100 every hour. The difference between his actual hourly rate (his hourly rate based on thousands of hours of play) and his hourly rate over the short term is called variance.