sample of data that is normally distributed, the mean should be close to the population mean. but depending on the sample you took, the data might be spread out compared to the mean, and thus your sample mean might differ compared to the population mean. as in the above example, if we took the times of 50 people running a 100-meter race, we might notice the mean is 16 seconds. if you took a sample of 100 people, the mean might be 15 seconds flat. the population mean would be closer to 15 seconds, as opposed to 16 seconds.
the larger the sample, the closer your data set will be grouped 