Reasons for variability in the response

In a completely randomised experiment, there are two potential reasons for variability in the response.

The different factor levels
Changing the value of a factor can result in changes to the response. The purpose of the experiment is to assess the magnitude of these changes. This is called explained variation since it can be explained in terms of the explanatory factor.
Other differences between the experimental units
There is usually natural variability between the experimental units and in the measurement process. Two response measurements at the same factor level will usually differ. This natural variability cannot be explained in terms of the factor and is called unexplained variation.

The distinction between explained and unexplained variation is critically important.