We're talking and listening to each other. I can't just talk and listen to myself, and I can't just talk and have all of what I say sink into everyone else without talking being directed back, to modify it. All this talking is shared in a crucial way that's similar to playing music together.
Murk happens when we hide all the stuff we do, pretend to do, or fail to do in making any of this happen successfully. In fact, I'd say role-players spend a lot of time hiding what they do, especially from themselves, to the extent that they do not know how they make anything happen. And when they don't know how, they don't do it well, either.
The cannibal king muses, 'Boy, I wish I could marry that hot princess from the neighboring island," as the tribe cook eyes you eagerly.
So, uh, this means ... what? A signal for you, saying you, the player, must now offer to act as go-between to the other tribe, or we cannot play further in this scenario? Or does it mean, the king isn't interested in you, and you better fight now or your characters are going to get killed and eaten? Or does it mean, I sure like playing this ditzy, romantic cannibal chief, and I'd love to see how you guys riff off him, no matter what happens?
The first is a signal that I want you to receive, to volunteer to go fetch the hot princess or carry a message to her, or something like that. The second is an equally directed signal that I want you to do something else, i.e., move to the combat rules now. The third is an opening for you to do whatever you like, no more and no less. But in each case, the sentence itself gives no indication which one.
Murk-ridden play means that none of us have any idea. We muddle through it like we muddle through everything, including endless wrangling over whose character is standing where, and whether this character had a weapon ready or not, and whether he could or could not see one or another of the foes. And we don't even know when we're supposed to switch into the hard-core combat rules or any similar subroutine mechanics - sometimes we hardly do anything and it's "roll initiative!", and other times we try to start a fight and the GM just keeps talking as his NPCs.
A core contributor to Murk is muddling up Authority.
By far and away, endless wrangling. When anything happens, people shout out where their guy is, jump into announced actions, carp about conditional or situational details without announcing actions, negotiate over who could or could not be doing what they're doing, object to input that was just provided ... sometimes to the extent that sorting out the cacophony and and moving into actually playing out what happens can occupy the majority of time spent. In other words, most of playing is actually not play. This is unfortunately so common that some forms of role-playing can accurately be described as "twenty minutes of fun packed into four hours play!" (Ryan Dancey's phrase, if I remember correctly.)
Even worse, people can develop skill at exploiting the Murk for individual advantage and/or control over the outcomes of events. Certain applications lend themselves to exerting Force, often its negative side.
A longer list of threads about Murk is listed here: Murk, or beating down confusion