VP9 is an open source, royalty free video codec. It is developed by Google to provide a free alternative to things like H264 and H265. Implementing codecs for both of those require paying licence fees.

Codecs can be in software, but can also be implemented in hardware which is much more power efficient. This change enables the Linux kernel to use hardware VP9 decoders so that software can decode (play) VP9 video much more efficiently when that hardware is available.

It looks like it implements hardware acceleration of the VP9 codec for some specific hardware (Rockchip VDEC and Hantro G2). This opens up playing, for example, lots of YouTube videos with less CPU usage on devices with that hardware. I can't comment on whether or not it "took so long" as I have no idea which hardware this is.

The title makes it out to be something fundamental in Linux, but this is just one driver becoming more complete.