A statistic is a measure on the items in a random sample. Since the only reason to ever draw a random sample is to infer something about the population from which it came, it should be clear that when we calculate a given statistic we only do so in order to estimate a corresponding parameter of the population from which the sample was drawn. An example of a statistic is the mean (i.e. average) of the measures in the sample.
As indicated by the central limit theorem, the mean of a random sample can be used to estimate the mean of the population from which the sample was taken.