"11th dimensional chess" is a term used to describe someone whose planning and actions are deeper than they appear at first. The derivation of the term comes from the fact that while chess is a complicated game, variants of it in higher dimensions would take such a fine strategic mind that their actions would appear random, confusing or even foolish to the uninitiated. The term has some variants, with the number of dimensions varying, or somethings just "nth dimensional chess", but the idea is that the strategies and subterfuges involved are so complicated as to be opaque to an observer.
Of course, like much new slang, the meaning is a bit more complicated than that. Because, much like people being "woke", the idea of people playing 11th dimensional chess is usually used in the negative, or as a piece of sarcasm. When a celebrity or politician or business leader launches into a series of bumbling or flailing moves, they are either sarcastically referred to as "playing eleventh dimensional chess" or just explicitly stated to "not be playing 11th dimensional chess". Sometimes, train wrecks and dumpster fires are just train wrecks and dumpster fires.
Are some people actually playing 11th dimensional chess, or at least are some actions more textured than they appear at first? Are you dealing with a real Zhuge Liang or Londo Mollari? Undoubtedly so, but if we ascribe every mistake to a master plan, then we basically have a blank check to describe every action as part of someone's long con. At a certain point, we reach the point of having no way to falsify our guesses: because any theory about hidden motivations or strategies means we can always just invent another layer of subterfuge to explain events.