The Meerinese Knot refers to the problems he's been having with the Daenerys chapters.
Originally, Martin intended to have a 5 year Time Skip
in the series. However, when he got to the point where the time skip was supposed to occur, he realized that things would not realistically stay on hold for 5 years and that going ahead with it would require so many flashbacks, explanations, or plot holes that it wasn't going to work. Note that he realized this after
publishing Feast, so his note at the end that Dance was upcoming was entirely truthful from what he knew at the time.
So, he had to scrap his timeline for the series and rework it for the new timeline. This added entirely new problems. The improbably low age of the characters and the small amount of time they have to gain the skills needed is one. Another is getting Dany to Westeros (which is apparently the biggest part of the aforementioned Knot).
In short, the reason Dance took so long is A) he had to almost completely re-write it and B) he had to re-outline the rest of the series before he could finish it. Add in that the man is a perfectionist to an absurd degree, and you get the gap between Feast and Dance.