Pop_OS is based on Ubuntu. It’s developed by System76 which sells linux laptops that run their distro by default so it’s very well maintained and polished.
It’s a popular recommendation specifically for people looking out to try gaming on Linux because there are specific features built in like performance improvements for gaming and some gaming-specific packages whereas Ubuntu, Mint, Fedora and OpenSuse are generally designed to be a general purpose distro. Pop_OS delivers packages as flatpacks by default as opposed to Ubuntu’s snaps which are a bit controversial and also uses their Cosmic desktop environment by default (though as far as I know gnome, kde, xfce, etc all still work fine if you have a preference).
Mostly I recommend Pop_OS for people that are new to Linux, don’t know about why they might prefer one distro over another, and want to try it out with the minimal amount of hassle. If you aren’t gaming Pop_OS is still great but that’s one of it’s selling points.