In this example, we are testing 2 Factors for their effect on the y Variable, Cleanliness.
We see from the graph that -- for all three levels of Factor B, Detergent #1 cleans better than Detergent #2. The lines are substantially separated, indicating that the difference is Statistically Significant. (The ANOVA numbers will tell us for sure.)
If Factor B did have an effect, the lines would be slanted. Again, separated lines tell us that Factor A has an effect.
If the lines were not separated, as below, then Factor A does not have an effect.
Andrew A. (Andy) Jawlik is the author of the book, Statistics from A to Z -- Confusing Concepts Clarified, published by Wiley.