DeGoogle Yourself

It means Carrier unlocked. If it's carrier locked (depends on where you're from, what carrier it came from, and/or if it came from a plan upgrade), you may not be able to install GrapheneOS on it unless you get it unlocked first.

If you get a second hand one, make sure the person you're buying it from is willing to help you unlock it.

First question in their FAQ: https://grapheneos.org/faq#supported-devices

Devices sold in partnership with specific carriers may be locked by the carrier, which will prevent installing GrapheneOS. This is primarily an issue with US carriers and isn't common elsewhere in the world. To avoid this, either don't buy a carrier device, or make sure it can be unlocked. It's the same hardware/firmware/software either way but carriers dislike having devices able to bypass their paywall for tethering, etc., so they disable it for the devices they sell as part of contracts.

Get it from the Google store

You could also take a look at e/os or lineage which can be installed on like up to 14 year old phones and support more than just pixel
But it might be not what you want so better inform yourself before deciding

when people talk about unlocked phones, they mostly refer to the bootloader being unlocked. in that state the bootloader allows "flashing" (overwriting) the phone's partitions, and that's what you need for replacing its operating system.

as I know almost all phones usually come with locked bootloaders, which is not bad because it provides security at least while it is on the way. but normally they can be unlocked. some manufacturers require custom account login and a working sim card for that, samsung started taking that away recently even retroactively for older phones.

the carrier lock is a different thing, but if your phone is carrier locked, or it is a carrier variant, usually you won't be able to unlock it (by yourself)