The probability value (p-value) of a hypothesis test is the probability of getting a value of the test statistic as extreme, or more extreme, than the one observed, if the null hypothesis is true.
Small p-values suggest the null hypothesis is unlikely to be true. The smaller it is, the more convincing is the evidence to reject the null hypothesis.
In the pre-computer era it was common to select a particular p-value, (often 0.05 or 5%) and reject H0 if (and only if) the calculated probability was less than this fixed value. Now it is much more common to calculate the exact p-value and interpret the data accordingly.