Standard error
For example, there are many redfish in the water of the Gulf of Mexico. To find out how much a 42 cm long redfish weighs on average, it is not possible to measure all of the redfish that are 42 cm long.  Instead, it is possible to measure some of them. The fish that are actually measured are called a sample. The table shows weights for two samples of redfish, all 42 cm long. The average (mean) weight of the first sample is 0.741 kg. The average (mean) weight of the second sample is 0.735 kg—a little bit different from the first sample.  Each of these averages is a little bit different from the average that would come from measuring every 42 cm long redfish (which is not possible anyway).