It is literally a thing. I cannot for the life of me make sense of how to make it work that doesn't strike me as off, but enough people get utility out of the moniker that it's got a page. So...
Tax loss harvesting is real, you just time when you take a loss to offset a gain. Then immediately reinvest. You haven’t lost any money in total, you can just claim the loss on your taxes (but will pay higher capital gains in the future).
Tax are due on capital gains which is basically <sale price> minus <purchase price>.
You can't claim a capital loss unless you paid the overvalued price. And then you just lost money anyways.