For example, apples are cultivars because if you grow an apple seed from a delicious apple you eat, the apples that get produced from your tree won't taste anything like the original. Orchards get around issue with growing tasty apples you love to eat by taking 'cuttings' (which are in essence a clone copy of that tree) from the parent tree and growing the cuttings into full mature trees.
Now I'm not saying grow apples, but look at interesting plants/trees that interest you and what you want to accomplish. Do you want to improve make a fruit more tasty? Faster growing plant? More medically potent effect? Etc.
> In short, a cultivar is a plant that is produced and maintained by horticulturists but does not produce true-to-seed; whereas, a variety is a group of plants within a species that has one or more distinguishing characteristics and usually produces true-to-seed. [0]
[0] - https://extension.unl.edu/statewide/buffalo/Yard/Cultivar%20...