Ultimately, it’s just a common proverb. The revelation of the manuscript is designed to illustrate that Jack - despite the sound of lots of typing, as if the writing was going well - hasn’t actually written a single useful word … and is not only mad as a brush but has been for a while, just repeating the same line over and over and over.

Obviously, because it’s Kubrick, people always look for deeper meaning. For example, one theory is that Jack has been “free-writing” - a technique used by writers to try to break writer’s block, where they type out whatever’s in their head, like a stream of consciousness.

Free-writing pre-dates the typewriter and was originally done by hand using a pen-holding device called a planchette, so you could just move your hand and form whatever words or pictures were in your head…

… a system that was adapted into a game known as Ouija, which simply used the planchette to point toward pre-written letters and words.

So, if you’re willing to entertain that theory, then it could be argued that Jack has effectively been playing Ouija and that’s how the ghosts of The Overlook have got into his head, where they’ve been urging him to take a break from the work of trying to write and have a little play instead.

Interestingly, for the dubbed foreign-language versions of “The Shining,” Kubrick filmed different inserts for the manuscript reveal. He didn’t just translate the same “All work and no play makes Jack a dull boy” into those languages, but substituted different proverbs from those languages.

English: “All work and no play makes Jack a dull boy”

Italian: “Il mattino ha l’oro in bocca” (lit. “The morning has gold in its mouth” - equivalent to “the early bird catches the worm”)

German: “Was du heute kannst besorgen, das verschiebe nicht auf morgen” (Never put off until tomorrow what can be done today)

Spanish: “No por mucho madrugar amanece más temprano” (lit. “getting up earlier doesn’t make the dawn come any sooner” - so, something along the lines of “things must take their natural course”)

French: “Un Tiens vaut mieux que deux Tu l’auras” (one today is better than two tomorrow, which means the future is not guaranteed so seize today)

So you’ve got 5 different idioms that Kubrick thought fitted. They’re all meaningful, albeit in different ways, which does seem to support the theory that Jack is being urged by the ghosts to (respectively) have fun, act now, don’t put off what needs to be done, let things take their course, and seize the day.