Android 10 isn't exactly a huge rethink of the most popular operating system in the world, but it does bring with it a wealth of improvements and refinements that make it easier to live with on a daily basis. The biggest change with Android 10 is the inclusion of a new systemwide gesture system, which allows you to swipe to go back, or swipe up to present the home screen (among other gestures). The goal with this new gesture system is to kill off permanent navigation buttons at the bottom of the display.
A new system wide Dark Theme lets you channel your inner Darth Vader at any time, or you can simply invoke it for individual apps if you prefer. Given that many Android smartphones use OLED displays these days, switching to the Dark Theme can also give you a bit of battery boost as the UI won't have to "light up" as many pixels on the display.
Smart Reply now has suggested actions that allow you to seamlessly launch a YouTube clip or navigate with Google Maps without invoking copy/paste, and there are a tons of new privacy- and security-centric updates to keep your data safe and protect your device from malicious attackers.
Android 10 also includes baked in technology to further leverage the incoming wave of 5G smartphones and the [currently stalled] launches of foldable Android smartphones from the likes of Samsung and Huawei. There's also a new Focus Mode that cuts down on distracting notifications and apps, while there are now more closely integrated family controls for parents.
If you currently have a Pixel/Pixel XL, Pixel 2/Pixel 2 XL, Pixel 3/Pixel 3 XL, or Pixel 3a/Pixel 3a XL running Android Pie, you can simply head on over to Settings --> System --> System Update --> Check for Update to get Android 10. You can also download system images directly here.