Chi-Square is a statistical term for an equation that tests the frequency distribution (the rate at which a measured event occurs) in comparison to the frequency distribution of another measured event. The chi-square equation is used make very fine distinctions between the expected probabilities of events against the actual measured occurrence of an event.
A common example of this type of distinction is the probability that exists when rolling one die. In this example, the probability of rolling any number between one and six is equal. In multiple rolls probability states that each number will come up an equal number of times, however the chance is that some numbers will randomly appear more often than other numbers. The chi-square equation is used to measure this type of difference.