Schlager is German-language popular song, and there is no real English equivalent. It is sentimental, melodic, built on simple structures and enormously popular, with its own stars, its own television shows and its own charts, almost none of which cross national borders. Any serious look at German radio has to deal with it, because it occupies a large share of the dial.
The modern form has absorbed a lot of dance production, and the boundary with German pop has largely dissolved. Discofox, which has its own tag here, is the partner-dance variant: the same repertoire at a tempo suited to a specific social dance that fills ballrooms across Germany, Austria and Switzerland every weekend.
Schlager stations are also the best available answer to what mainstream German radio sounds like to a German audience, as opposed to what the German stations aimed at international listeners choose to play. If you are learning German, the lyrics are unusually clear and unusually simple, which makes these stations considerably more useful than chart radio.