If you grow a vegetable/fruit grown from a hybrid seed then the seed obtained from the crop can not be re-used, you must buy the seeds again. This is the reason most commercial sellers sell hybrid vegetable seeds so that gardeners come to buy again and again. If you sow the seed from a hybrid crop, the resultant plant will never be as good as the parent.
Heirloom vegetables are cultivated forms of crops that have been perpetuated by gardeners who save seed (or propagate by some other means such as taking cuttings) from year to year. Some heirloom vegetable varieties have been around for more than a century! Gardeners have kept these varieties growing for generations because the crops performed well in a particular area or because they have outstanding flavor, unusual color, or other appealing characteristics.
OP stands for open-pollinated, meaning that wind, bees, or other insects, rather than plant breeders, transferred the pollen to fertilize the flowers. While all heirloom vegetables are open-pollinated, not all OP vegetables are heirlooms, since most seed companies offer modern-day varieties of vegetables that have been pollinated by wind or other means.
Seeds from the crop of Heirloom/OP vegetables are true to their parent. So you do not need to buy the seeds again and again.