Statistic - a measurement derived from measuring a sample. Then we use that sample to infer the pop. Random sample - Each member of the population has an equal chance of being selected. Sampling Error - Random samples are the best there are. So measurements of a sample probably never exactly match the same measurement if we could measure the population.
Population mean is mu, sample mean is X bar.
Sampling error is the difference between the sample value and the population value. Sampling error is random - that means it could go either way. Just as likely to be higher than the pop as lower. Take 100 measurements of a sample. 50 should be higher than pop.