Answer:
Let's say we want to test the effect of five pH concentrations (highly acidic, slightly acidic, neutral, slightly basic, and highly basic) on an enzyme catalase.
We will add a fixed proportion of enzyme catalase in a reaction at following pH levels: 3, 5, 7, 9, 11. The pH will be adjusted by using acids and bases and tested via pH meter. If the reaction rate of the enzyme is 10 minutes, which is visible with color change for example, we will monitor the initial and final changes. In the end, we will realize that the highest change in reaction (e.g. change in color monitored through spectrophotomter) took place at x level of pH. All other changes will be less efficient. This can also be plotted on a chart (the numerical values). To put it further into a perspective, I have attached a plot where you can see that maximum change in reaction will occur at optimum pH. From here, you can see the relationship of pH and reaction and we can also find the optimum pH for the maximum reaction.