Z-score in statistics are a measure of how many standard deviations below or above the population mean a raw score is. A z-score is also known as a standard score and it can be placed on a normal distribution curve.
Z-score is used to determine the probability of a particular result occurring if the distribution is normal. For example, it is often used to confirm that the result we see falls within expected values and thus accept the null hypothesis when we do testing.
Farther away the particular result is from the probability assumed by the z-score, the more likely it is that the null hypothesis is not correct.