Most seeds are hybrids. This does NOT mean genetically modified. This means that a breeder has crossed two different varieties of a plant, and come up with a desirable child. The problem with hybrids are the child seeds are unpredictable. Let's say you crossed a sweet pepper and a very hot pepper. The resulting pepper may be as sweet as the first, but have the shape of the second with no hotness.
The child seeds of this hybrid may be of all types. For example, one might be sweet only, the other one hot, the next bearing the shape of the second. In other words no reliability, no consistency.
A heirloom has been bred and selected for many generations for similar traits. Because the heirloom plant has had grandchildren and great great great grandchilden all selected the same, it is very very likely, that generation after following generation will all share the same traits. That means you can save your seeds, and have reliable children plants. Which means you only have to buy the seeds one time.
There is also more variety in heirlooms. Since most supermarket vegetables are bred to have similar traits, as most people are not creative, and expect all tomatoes to be red and a certain size and flavor. They also must be able to ship without splitting, and have a long shelf life, etc. Heirlooms can break all these rules, for example, yellow, huge, and extremely sweet, etc.