HDR means High Dynamic Range. The range of values on an SDR display (Standard Dynamic Range) is 16-235 but the range of values on an HDR display is 0-255, that means that the color “black” on an HDR display is actually black and not “dark-gray”.

This is dumbed-down because I don’t know the more technical stuff.

Also it has nothing to do to “color saturation” (wrongly called “vividness”). That’s a dumb marketing thing since you can do the same using every display in this world.