What are association neurons?

I think you could mean association fibres - the fibres (also called axons) being the long thin part of a neuron? Association fibres connect various areas of the brain to one another, and act as a kind of relay station where incoming sensory information is compared with previous experience of that information, and then integrate the appropriate motor response. For example, imagine you're driving along the road when you see a traffic light up ahead turn to amber. So the sensory input is visual information from your eyes; it arrives at the appropriate 'relay area' in your brain, where information is quickly brought up from the learning areas reminding you that an amber light means it's gonna be red by the time you reach it, so you'd better start slowing down; another visual message supplies the information of how far you are away from the light, and yet another how fast you are driving; now some more 'learned' information comes in, telling you (based on your previous driving experiences) things like how many seconds it'll be before you reach the light, and how hard to need to press on the brake pedal to be able to stop safely and in time. In the meantime, messages have been drafted & sent to the motor cortex: the rough draft is that the muscles in your leg & foot are gonna have some work to do very soon; the motor cortex responds by sending a message back to the association area: "OK, ready when you are; this is the current state of play as regards the precise position of foot and leg right now - so how hard are we gonna have to work?" So now the association fibres have all the information they need; they integrate all of these messages so that they the final message sent out via the motor association area to the motor cortex can tell those muscles exactly how hard your foot needs to press on the brake pedal to stop the car in the precise spot it needs to stop. This is a very simplified description of the incredibly complex work done by the association fibres to help you carry out just one little action.