Wefwef is an an web app, like an app, but that runs powered by your phone’s web browser, usually accessible through a URL and you can then install it.

m.lemmy.world is just a page, hosted by lemmy.world. Typically, it should have just had a mobile view of the usual lemmy frontend. But what this announcement is saying is that Wefwef was made the official mobile frontend for that website, and thus, going to that page will show you the contents through a wefwef view.

Also replace all places where I wrote wefwefs with voyager, since they rebranded.

I assume the idea is to give people an easy and nice looking default mobile experience for lemmy users without users having to go find things like wefwef/voyager themselves.

There is no difference, it’s two websites hosting the same software. The idea I think is to use wefwef as the mobile view for lemmy.world, since you can also use it straight from your browser (the beauty of a web app!).