To extend the explanation, all examples of a named cultivar can be traced back to a single seed. (We'll skip sports, brooms, and mutations for the moment). So all Acer palmatum 'Bloodgood' we're propagated by cuttings or grafting from a single seedling. They are all genetically identical.
If you plant a dozen seeds from 'Bloodgood', each seed that sprouts is genetically different from 'Bloodgood', as 50% of genetics came from a different tree. In the case if self pollination, the pollen only has 50% of the 'Bloodgood' genes, and the ova (seed,egg) only has 50% of the genes of 'Bloodgood'. Key is, the 50% is a random mix of the dominant and recessive genes that makes 'Bloodgood'. If you raise a batch of say 100 seedlings from 'Bloodgood', you will get to seed perhaps a dozen different recessive traits not expressed in 'Bloodgood'. Hidden colors, leaf shapes, bark texture, growth rates, disease resistance, winter hardiness, and many more traits will vary in the seedlings from a named cultivar. Some might look a lot like 'Bloodgood', but will have some differences.
If you raise a a batch of seedlings, and one is unique enough that you propagate it, by cuttings, grafting or tissue culture, it is considered a cultivar, and you may name it. If it is commercially popular, you may even register it with the RHS, or trademark it, or patent it. Only patent gives you royalties, trademark is brand protection, RHS registration is merely bragging rights, but can be used to support patent rights.
Now you have half of what you need to know about cultivars, and I'm tired of typing, and I bet most readers abandoned this post back at the second sentence.