Your eye can only really see detail in a very small area where you are directly looking (in the centre of your vision), but your brain is very good at filling in the blanks and hiding this fact. It drops off extremely rapidly, and for the vast majority of your field of view, you can resolve barely more than basic colour and movement.
The idea behind foveated rendering is, you use eye-tracking to figure out where the user's eye is looking, and then you render a very small full-detail image and place that where they're looking, and then you render a larger lower resolution image and put that behind it, repeating that until you're rendering very few pixels out around the periphery of the vision. Your eye can't tell that the image is getting less detailed (blurrier, really) as it moves away from your centre of vision, because you can't perceive the lack of detail. Obviously I'm simplifying with the progessively-lower-resolution-images description, but that's the gist of it.
When I say the detail drops off really fast from the centre of your vision, I mean it drops off EXTREMELY fast. Check out this graph:
Just 10 degrees off centre and you can only see 20% of the detail. But current rendering systems would still be drawing 100% of the pixels in that area.
To get an idea about how much processing power you can save with foveated rendering in an ideal case, basically consider the area of that graph overall (the whole square) versus just the area under the line. That's a speedup of multiple times.
I'll qualify all this with, I'm no expert, I've just read into it a bit.