I'll note for conceptual understanding (by people who aren't patio11, who I'm sure understands this all far better than me) the general point that a gain or loss on the sale of a capital asset is how much money you got from selling it minus how much you "paid". More technically, how much you "paid" is called the basis, and you can do things that adjust the basis as patio11 noted above.

For general information on this topic see https://www.irs.gov/newsroom/capital-gains-and-losses-10-hel...