You can view what’s new in the Feature Drop portion here, but a few of the highlights include a battery widget, a big update for At a Glance, Live Translate to Spanish, plus a whole lot more. It’s actually a pretty sweet update, coupled with a huge list of bug fixes. It’s a very important update and you should flash it ASAP.
As for the build number, Google lists it as SP2A.220305.013.A3 for both the Pixel 6 and 6 Pro.
Google should begin pushing these updates shortly over-the-air (Settings>System>Advanced>System update), but if you don’t want to wait for Google and prefer to update manually, you will find each factory image or OTA file at the links below. For instructions on how to flash a factory image, here you go. For instructions on how to flash an OTA .zip file, here you go.