The tone curve in Lightroom is one of the less understood tools; nonetheless, it's one of the more powerful ones. It can be a bit tricky to figure out on your own, but this tutorial is a perfect way to get acquainted or further acquainted with the tool.
The tone curve is one of the more popular tools used in Lightroom for those out there creating and selling presets, because it's much harder to copy than the other controls, which are simply numeric sliders. Regardless what your motive is for learning, understanding, and using the tone curve, it helps to have it explained in such a way like how Julia Trotti does in this video. The tone curve can be an insane amount of fun once you get the hang of it. It can also be an incredibly deep rabbit hole, simply because it's more difficult to undo, redo, or replicate ways you've used it.
I have found myself, more than a few times, starting from scratch with it simply because my edit ends up being something that I don't like, and the curve is way too involved to attempt to reset it to a state where I actually liked it. I don't, however, see that as a bad thing. It's all part of my own learning process. Usually, if I fail with an edit, then my very next one is infinitely better, because I scaled back my approach to a much more reasonable process. Give it a shot if you haven't already; it's quite a lot of fun.