It is a "copyleft" software license. So basically its a copyright that states you can freely distribute and modify the software, but under the restriction that any modifications also be open source and published under a GPL license

So really cool example of this license working, is the Linux kernel is under the GPL so when steam wanted an OS (steamOS which is actually based on Debian) they got the benefit of not having to build an OS from scratch, but under the restriction that their OS had to be open source (at least the kernel anyway)

https://github.com/ValveSoftware/steamos_kernel?tab=License-1-ov-file

You can learn more from the free software foundation:
https://www.fsf.org/

Or look up Richard Stallman and listen to his reasoning for creating the GPL