Billy here (known as Shazam, previously known as Captain Marvel) is basically just a child with the ability to transform into a big superman-like hero. Here, he just hit puberty and wants to have... er... "private romantic moments"... with Power Girl, who is more notably known as this strong kryptonian who likes for some reason to show her breasts a bit, and that's a running joke that has been used in the comics for a while.