responsive design: do pixels even mean anything?

[cellio]

I don't know a lot about the nuts and bolts of responsive design (the "how", I mean), but Stack Exchange is moving toward it so I'm starting to pay attention.

Meanwhile, my ancient tablet seems to be in its death throes, so I've started to look around at what's out there these days, and I realized something. I'm looking at some 10" tablets with resolutions like 2048x1536. My 30-inch monitor at work is something like 2500px wide. These are, of course, not even remotely the same size pixels. Pixels have always varied with the size of the monitor, of course, but a ~10" tablet used to be in the range of 1024 or 1280 wide (landscape), not twice that.

I've seen discussions of SE's upcoming responsive design that say things like "and at widths under 900px it does this" and "the max width for the content area is (some number of pixels)".

How does this work? How can I see reasonable "real-world" sizing of things on both my big monitor and my tablet when designers are measuring things in pixels and tablets are doing crazy-dense things with pixels these days? I guess the same can be said of 4k displays (which I don't have). Do these ultra-dense devices somehow tell the browser "no, really, treat me as half that for layout purposes"? On a tablet will I need to have tons of zoom -- but still struggle to see the actual application's controls, because those don't zoom when you make content bigger?

I must be missing something obvious. Anybody want to enlighten me?