A p-value is a statistical measure that indicates the probability of observing a result as extreme as, or more extreme than, the one that was observed, assuming that the null hypothesis is true. In other words, it is the probability of getting a result that is as extreme or more extreme than the one you observed, assuming that there is no real effect.
P-values are used to test the statistical significance of a result. A low p-value (typically less than 0.05) means that the result is unlikely to have occurred by chance, and therefore provides evidence in favor of the alternative hypothesis. A high p-value (typically greater than 0.05) means that the result is likely to have occurred by chance, and therefore does not provide evidence in favor of the alternative hypothesis.