A fundamental principle in science is that when we measure a quantity repeatedly, the result that we get varies. Although this principle applies to all branches of science, it is the most noticeable and important in biology. This is because biology deals with living organisms and living organisms are by their very nature variable.
Let's say that the quantity which we want to measure is "the mass of the house mouse, Mus musculus". Even a non-scientist would realize that we would be silly to measure a single mouse and assume that the mass we measured would apply to all mice. Obviously, a juvenile mouse would have a smaller mass than an adult. Females might weigh less than males, or more if they were pregnant. Individual mice may weigh more or less depending on their diet in the recent past. The differences that we observe if we make measurements of multiple samples is called variation.
We could reduce the variation in our measurements if we defined the quantity that we are trying to measure more narrowly. For example, we could measure "the mass of adult male mice captured in the Vanderbilt football stadium". Even so, we would still expect some variation because of genetic differences between the mice and small differences in their environment as they developed.
The amount of variation in our measurements would also depend on the method that we used to make the measurement and the instruments that we used. For example, measuring the mice in different Dixie cups on a postal scale would introduce more variability than measuring them in the same container on an electronic analytical balance. It is also possible that errors in technique can contribute in a minor way to variation in measurements, but it is a common mistake of naïve students to attribute variation in measurements primarily to errors. All biological quantities are simply variable by nature and one of the major jobs of experimental biologists is to assess that variability.