The P-value is the probability of obtaining the observed t-value or a more extreme value if there is no significant difference between the two groups being compared in a t-test (i.e., under the null hypothesis). A smaller P-value (typically less than 0.05) indicates that the observed difference between the group means is statistically significant and unlikely due to chance, while a larger P-value suggests that the observed difference may be due to random sampling variability.