Imagine you want a sandwich. In the kitchen you get a loaf of bread and take out two slices, since you’re making a sandwich only for yourself. You are focused on the two slices of bread and not even looking at the rest of the loaf.
While you’re making your sandwich, a group of friends drop by - ten friends and they’re all hungry for sandwiches. Now instead of making a sandwich for yourself, you’re making eleven sandwiches so you’ll need 22 pieces of bread. Now you’re focused on the entire loaf of bread instead of just two slices.
When taking a portrait, you want to focus attention on the person while blurring the background behind the person. By shooting at f/2.0 the DOF will be shallow. Just like the loaf of bread your focus will be only on the single person. Two slices of bread is like f/2.0 - they’re both meant for just one person.
If instead of a single person you’re photographing a group of people, who are standing in several rows, you’ll need a deeper depth of field. By shooting at f/22 you can increase the DOF and everyone will be in focus. Twenty-two slices of bread is like f/22 - they’re both meant for a group of people.
When setting your aperture to impact the depth of field, remember to think about the loaf of bread. Are you focusing on enough bread for just one person, or focusing on the whole loaf of bread?
Note: Thank you to Christopher Balmer of Perfect Light Camera for introducing me to this metaphor.