Availability Heuristic refers to how easily something that you've seen or heard can be accessed in your memory. People tend to think of things they remember as more important than things they don't remember as easily. A romantic relationship may grow because a person you've seen comes to mind after you've left them, leading you to assume this person must be important. In the same way, new friendship possibilities might have been overlooked because a person you've met several times has never seemed familiar, or they didn't make an impression on you.
This phenomenon is what makes the marketing process of "branding" work. A company makes a slogan or logo and shows it to you over and over again, until that company comes to your mind easily. You may not have any particular preference for the brand, but the marketing people count on your familiarity with the name alone to sell the product.