The tone curve can seem a bit intimidating at first, but once you get the hang of it, it's by far one of the most intuitive, efficient, and powerful ways to adjust your images. This great video will introduce you to both how the tone curve works and how to use it.
Coming to you from Signature Edits, this great video will show you the principles behind the tone curve in Lightroom and how to use it to edit your images. The main idea behind the tone curve is that the horizontal axis contains the full range of standard luminance values from black to whites, while the vertical axis contains the target range. To adjust a certain luminance area, you find it on the horizontal axis, then drag the corresponding point on the curve to the target on the vertical axis. It can feel a bit mathematical and overwrought at first, but once you get used to it, you'll probably prefer it over sliders. You can also use it for quick color toning by using the curves for individual channels. It's a tool that every photographer should definitely know. Check out the video above for the full rundown.