Four heads, four different realms to watch over. To what realms he watched over exactly is hard to piece together, but what I understand to be:
The underworld of the dead
The world of eternal spring and greenery where the birds flew in the wintertime, where Weles resided
The world of the living, our place
The world of the gods
His four heads watched over these worlds, a quiet ruler. Sources assume he is a form of Perun, and was worshipped with similar ferocity.