It's basically a really well polished app to browse the website on Android. It has a lot of features and also very good customization options since it kept adding features over the years.
Having a Lemmy client that good will make it much easier to use on a phone.
It's just a patch in the Manager. You may need to download the free version APK from somewhere like APKMirror and patch that though (you can also disable ads so it's like the Pro version), as there don't seem to be any Pro APKs and you can't patch an installed app without root.