Justin Alexander gives the following brief definition, before trying to come-up with something more precise:
An associated mechanic is one which has a connection to the game world. A dissociated mechanic is one which is disconnected from the game world.
What it all comes down-to is immersion. Dissociative Mechanics tend to encourage Rules-Centric gaming and ultimately hurt immersion. It's not a direct relation. You could have a World-Centric game with dissociative mechanics, or a Rules-Centric game with associative mechanics. The point is what the rules tend to encourage.
The more things going on in the game that have no intuitive mapping to the game-world, the more players are drawn out of the game-world.