In ANOVA, factors are either fixed or random. In general, if the investigator controls the levels of a factor, then the factor is fixed. On the other hand, if the investigator randomly sampled the levels of a factor from a population, then the factor is random.
Suppose you have a factor called "operator," and it has three levels. If you intentionally select these three operators and want your results to apply to just these operators, then the factor is fixed. However, if you randomly sample three operators from a larger number of operators, and you want your results to apply to all operators, then the factor is random.
In Minitab, different ANOVA commands can handle different types of factors. General Linear Model and Balanced ANOVA can analyze both fixed and random factors. However, Fully Nested ANOVA requires random factors while Two-Way ANOVA requires fixed factors.