The “power” (denoted π) of a hypothesis test measures how likely the test is to detect an effect, if the effect does in fact exist. It is the probability that the test correctly rejects the null hypothesis when some specific alternative hypothesis is true, that is, 1 minus the probability of a false negative.
Higher power means you avoid false negatives, but also means you may detect effects at sizes that are so small as to be practically insignificant. π = 0.80 is a commonly accepted threshold.