You could do with reading the first series, Rue Britannia, first, for a general understanding of what the Phonomancers (people like Penny in the scans above) do. To put it in the simplest of terms, their appreciation and understanding of music allows them to perform acts of magic (and that is putting it extremely simply, the creators would probably stab me for doing that).
The first series follows a single Phonomancer in his efforts to find out why an aspect of a goddess, Britannia, is being tampered with, and naturally, given she helped him become what he is toda, any tampering with her tampers with him. This story gives you a basic idea of what a Phonomancers are capable of, but it was always implied there's more than one kind.
This series revolves around seven different characters on one particular night, focusing on their particular experiences and how they interact, and in general, expanding the world of the series. This first issue alone reveals a completely new type of Phonomancer in Penny, who apparently uses her dancing as part of her magic.