The chi-squared test is used with categorical data to see whether any difference in frequencies between your sets of results is due to chance. For example, a ladybird lays a clutch of eggs. You expect that all of the clutch will hatch, but only three-quarters of them do.
Is the failure of some of the clutch to hatch statistically significant, and if it is, what could be the reason for it? In a chi-squared test, you draw a table of your observed frequencies and your predicted frequencies and calculate the chi-squared value. You compare this to the critical value to see whether the difference between them is likely to have occurred by chance. If your calculated value is bigger than the critical value, you reject your null hypothesis.