The 6x6x6 Rule (or 3x3x3 Rule) is an application of Miller's Law (https://lib.hol.ink/index.php?topic=29.0) used when chunking information during study or when developing a creative process or technical procedure.
To keep the maximum strain on our working memory under 9 items, we try to organize practical skills into sets of six no more than three layers deep. At the third layer, we're dealing with six items, plus three layers of depth. 6+3 = 9.
For beginners to a skill or those with ADHD, it can help to stick with a 3x3x3 approach, so they aren't pushing that working memory past 6. That gives three items in the current list and three levels of depth. 3+3 = 6 slots in working memory being used.
The full tree of an area of expertise may be much deeper (though preferrably no more than 6 levels if possible, so the full depth can at least be conceptualized with a few children scanned at a time). But the branches you need to actively use during a process should be restricted to a depth level that you can reliably traverse without losing your place in the tree or experiencing undue cognitive load (https://lib.hol.ink/index.php?topic=31).
This rule applies to not only your own studies and processes but also the processes you are putting others through.
Like when presenting information to a viewer in graphic design or instructional design, chunking the core ideas or steps into sets of 6 makes them easier to digest. There is a popular belief that a slideshow having 6 bullet points with 6 words each is enough to apply this rule, but even that much information can be enough to induce cognitive overload if the presenter is discussing other ideas verbally, as it's the combination of everything that's going into their working memory.
Of course, this also applies to storytelling (https://lib.hol.ink/index.php?topic=25.0) more broadly, as you are guiding your listener through experiencing unfamiliar scenarios with new information.