iPhone X has a 12-megapixel ƒ/2.4 "telephoto" lens with optical image stabilization (OIS) as part of its back camera system. That's compared to a 12-megapixel ƒ/2,8 non-optically stabilized lens on iPhone 8 Plus and iPhone 7 Plus.
It's important because the tele doesn't just help gather depth data for Portrait Mode and Portrait Lighting, it handles the 2x optical zoom feature. And, when the camera system doesn't have enough light, it'll switch back to the wide angle and digital zoom to preserve image quality. (See Serenity Caldwell's explainer.)
So, how much difference does that really make? Apple says about a 1/3 of a stop in normal shooting situations. But if you check out the difference it makes in terms of the camera system switching from the 12-megapixel ƒ/1.8 wide angle lens to the telephoto, well, take a look at Dan Provost's test on Studio Neat:
Dual Lens comparison on iPhone X and iPhone 7 Plus from Studio Neat on Vimeo.
I also used the manual camera app Halide to get an ISO and shutter speed reading at the various light levels, and came to the conclusion that the iPhone X requires roughly 2 fewer stops of light before switching to the telephoto lens, as compared to the iPhone 7 Plus.
For most people, when the iPhone X camera system chooses to switch between lenses is an implementation detail. You should just be able to shoot and get great results and not have to worry about what's happening behind the curtain.
For camera nerds, though, this is great to see. And it prompts the question: Where and how far can Apple push the iPhone camera system next?
