A situation or problem that is more complex than it first appears, just as a strand of yarn is actually made of many smaller fibers.
A ball of yarn coming unwound can represent a situation that's coming unwound or "falling apart," or the idea of getting to the bottom of a problem or figuring something out.
A person winding yarn can represent putting things in order, trying to get things under control or organized (or trying to at least appear that way to others).