Secure boot is a process that uses cryptography and untamperable physical circuitry (just like in a Bitcoin hardware wallet) to guarantee your phone boots into GrapheneOS and not a fake/modified version of Graphene that your phone got 'infected' with. It's the highest level of security available on consumer mobile phones.

Only Google phones have an open secure boot process, so therefore GrapheneOS only works on Google phones. Other Android phones often don't have a secure boot process, and the ones that do are inconsistent (can't guarantee every phone has the same process, even within the same family of phones) and less secure. As a result, GrapheneOS can not work on non-Google phones.

You can run many OS on a Pixel and leverage secure boot, but Graphene offers the best security/privacy.

You won't find secure boot on non-Google phones, so you won't be able to install Graphene, but you can still run Calyx/Lineage which at least degoogle the phone.