True to seed, or growing true, refers to plants whose seed will yield the same type of plant as the original plant. Open pollinated plants, which include heirlooms, will almost always grow true to seed if another variety does not cross-pollinate them. For example, saving seed from a 'Brandywine' tomato should result in another 'Brandywine' tomato plant.
If cross-pollination occurs, the resulting seed will be a natural hybrid and may have some characteristics of each parent or may look totally different than either. So if by some chance your 'Brandywine' cross-pollinated with a nearby 'Green Zebra' tomato, you will get some interesting tomatoes from planting those seeds.
Many of the seeds commonly sold are intentional hybrids, crosses made between two different plants for some type of benefit or feature, like disease resistance, better flavor or quicker ripening. These seeds will never grow true.
Fruit seeds rarely produce a similar tree or fruit as the parent. That is one of the many reasons fruit trees are usually grafted.