The difference between means is thus 2.1 - 1.5 = 0.6 seconds. The test of whether this difference is significant consists of determining the probability of obtaining a difference as large or larger than 0.6 seconds given there is really no effect of magnitude of reward. If the probability is low (below the significance level) then the null hypothesis that magnitude of reward makes no difference is rejected in favor of the alternate hypothesis that it does make a difference. The null hypothesis is not accepted just because it is not rejected.