It’s a variety of plant that can only maintain its unique characteristics when bred by man. A cultivar will always be named and recognized by horticultural groups; some may even be patented.
Apple trees are a good example — though the species as a whole would continue without us, without generations of farmers reproducing the particular types of apple trees, those particular types would have long ago died out. Each type of apple is a cultivar, because apples will not reproduce true to seed. Many olive trees are also cultivars. Corn is as well.
The opposite of a cultivar is a “variety” — a plant variety that occurred in nature and could maintain its characteristics even if we weren’t around to look after it.
Most people outside of horticultural circles, however, will likely just refer to both cultivars and varieties as “varieties.”