Before you continue reading, note that foveated streaming is not the same as foveated rendering, though the two techniques can be used alongside each other. As the names suggest, while foveated rendering involves the host device actually rendering the area of each frame you're currently looking at with higher resolution, foveated streaming refers to sending that area to the headset with higher image quality than the rest of the frame.
It's a term you may have heard in the context of Valve's Steam Frame, where it's a fundamental always-on feature of its PC VR streaming offering, delivered via the USB PC wireless adapter by default.
Given that the video decoders in headsets have a limited maximum resolution and bitrate, foveated streaming helps prioritize resolution and compression quality where you're currently looking.