Explanation in Arkham Asylum: A Serious House on Serious Earth
The doctor elaborates on the condition. The Joker supposedly finds that modern life too overpowering and stressful. To deal with this, he restructure his mind and personality. As such, one day he could be a harmless goofball and a murder the next.
Why This Make No Sense
"Super-Sanity" is a good example of when a writer is writing about something he has no clue about. I know what you are thinking: "Grant Morrison, the comic's writer, intentionally made this incorrect to paint the doctors at Arkham as having no idea what they are talking about". However, in Grant Morrison's Batman and Robin, Joker says he is "just different sane" (referring to super-sanity) and Robin confirms this.
While "sanity" lacks an universal definition, it is generally defined as something along the lines of a person's ability correctly function in everyday life thus the Joker cannot be "super"-sane. Super-Sanity seems to actually be a case of extreme dissociation. Caused by an inability to deal with life's stresses or trauma, dissociation is a psychotic detachment from one's surroundings.
Later writers used the term. However, they altered its meaning. It now seems to mean that he is partly aware he is a comic book character. If Joker knows he is comic character, his psychotic behavior might his warped attempt to entertain his readers and reinvents his personality to meet up the current taste of readers.