A p value of less than or equal to 0.05 shows the result is statistically significant. It indicates strong evidence against the null hypothesis (no change in effect) as there is a less than 5% probability the null is correct (and the results are random). Therefore we reject the null hypothesis and accept the alternative hypothesis. It says there was a big enough difference in the before and after too show the masks made a difference. I'm not debating anything else of it, I think masks are bollocks, the study could also be bollocks, we just should be accurate that this clip, as presented says masks made it better, not that they made no difference.