It's Android with all of the Google removed where possible and sandboxed where not. You can choose to install the Google Play services and use it like any other Android phone or use it without any Google software.

Some things won't work, namely things like some banking applications and NFC payments, because they require on hardware attestation that Google will not allow Graphene to pass. Essentially everything that isn't banking/payment related works exactly like any other Android phone.

It is just a secure phone (though you can still install Facebook on it if you want) that is designed around mitigating attacks that could violate your privacy and security.

Very easy to install, you just buy a Pixel directly from Google (don't buy from the carriers, they'll be locked). Enable OEM Unlocking in the Developer menu and then plug it into USB and you can install it directly from the Graphene site via WebUSB. It takes about 5-10 minutes, then your phone will reboot (It'll give you a scary looking screen about not running a Google OS that you'll see every time it reboots but it's just informational, it doesn't affect anything and the system will boot into GrapheneOS in a second or two).

The more complete instructions and WebUSB install process:

https://grapheneos.org/install/