A primary key is a database terminology used to uniquely identify a row. For example, you might want to identify each student by their student ID. Problems can arise when you pick a primary key that turns out not to uniquely identify things. If you keep track of your students by their first names, you might find yourself confusing two students who share the same name!
In the same spirit, we have mental primary keys for identifying uniqueness between objects. I might say “the sushi in this michelin star restaurant tastes just like the one down the corner” or “isn’t what makes one cyclist better than another just how well they can pedal?” In the first example, I see the primary key of sushi as the freshness of the fish and the quality of rice. In the cycling example, I have a primary key that is the athleticism of the cyclist.