It is a punning reference to the phrase ‘trip the light fantastic’, which means (per The Phrase Finder)
To dance, especially in an imaginative or 'fantastic' manner.
The phrase seems to arise from the works of Milton, in Comus he wrote, as you have already seen,
Come, knit hands, and beat the ground,
In a light fantastic round.
And in L’Allegro
In-universe the "light fantastic" is an actual, factual thing.
There was no real need for the torches. The Octavo filled the room with a dull, sullen light, which wasn’t strictly light at all but the opposite of light; darkness isn’t the opposite of light, it is simply its absence, and what was radiating from the book was the light that lies on the far ...
The light that Pratchett refers to is Octarine.
This is defined in the Discworld books as the eighth colour of the spectrum and the colour of magic. "The Colour Of Magic" itself being a title of another book in the series.
This is fantastic because its existence is part of the Discworld fantasy universe. Pratchett is very fond of such puns and ...
One of the central themes of the story is how the protagonist's daughter closely resembles his wife at a similar age. He begins the story by describing his relationship with his wife, all the way from the beginning to now, particularly dwelling on the sexual parts. This segues naturally into discussing his daughter, whose teenage self he introduces into the ...
It's 'The Light(noun) Fantastic(adjective)'
That's how it is used both in Milton:
On the light (noun) fantastic (adjective) toe (verb)
and how this construction is generally rendered in English. Examples:
The Brothers (noun) Grimm (adjective)
The Brothers (noun) Karamazov (adjective)
In 1492, Columbus sailed the ocean (noun) blue (adjective)
A Tale (...
One example is Icelandic sagas. Two of the most famous such are Egil's Saga and Njal's Saga, bearing personal names. Others, such as Laxdæla saga have place names in their titles.
The examples of the Iliad and Odyssey from another cultural tradition show the same mix: place name and personal name, respectively, in this case.
In classical Greek literature ...
The title is obviously also a homage to, or play on, Rabbit Redux by Updike. The word redux has been more commonly used since its publication.
Wiktionary says this:
The word may have re-entered popular usage in the United States with the 1971 publication of the novel Rabbit Redux by John Updike, although it had previously been used in medicine,...