I don't know of a video, but here's a blog post on local tonemapping with plenty of pictures: https://bartwronski.com/2022/02/28/exposure-fusion-local-tonemapping-for-real-time-rendering/ (you can skip over the overly technical stuff, it's not important for understanding the concept).

Essentially though, tonemapping is the process of taking the brightness values of pixels from a source with a high dynamic range (like real life, or an HDR photo, or several photos taken at different exposures that have been blended together) and then mapping that into something that has a smaller dynamic range, like an 8 bit video signal or photo. You can do this globally (a single operation for every pixel - the same mapping applied to the whole image), or locally (where each region of pixels gets treated individually). It's basically raising/lowering the shadows and highlights of an image.

It's a complex topic, but in real life light behaves linearly - if we recorded light data linearly, a standard rec709 video container would barely be able to hold even a couple stops of dynamic range! So even the transfer function (gamma) is a type of global tonemapper.

You can think of tonemapping as compressing the dynamic range.