In many of the cloth uses of the mishkan it says that you should use שש משזר "a twisted 6". What is this? A 9 perhaps? What's the special significance of a twisted 6 and not just a 9?
A twisted 6 is actually an 8, not a 9, as the some of the other answers have suggested. It's pretty simple to see that. Just extend the top of the 6 and twist it around until it meets the bottom.
A twisted 8 is extremely useful when making a stein knot which is a form of a figure 8 (or twisted 6) knot. Wikipedia says:
It is used to secure a rope that is already passed around a post or through a ring. It is quick and easy to tie and untie. It is a device rigging rather than a true knot.
The curtains of the mishkan were fastened to rods, so, it made sense to shape them as 8's or twisted sixes.
It's an allusion to the third temple.
Ok, let me explain. First, it's a six, which is two threes, one for each temple we've already had. We know that a cord of three strands is not easily broken, and that R' Yochanan says the third strand is Hashem, Who is the reason we built the mishkan and temples at all. So "three" connects Yisrael to Hashem.
But sheish, six, tells us that three wouldn't be enough, and that we'd need to do it again -- two temples, so six cords.
But as this answer says, six really means nine. Which is three sets of three, so the two temples of the past and the one yet to come.
And that one will endure, because a cord of three cords of three surely cannot be broken.