It relates to being clear and understandable. For example you would need coherence between two people working on a detailed or complex project, so they both know what they are doing and what the other is doing and the overall objective.
Mean can mean you are being rude or in math terms it is adding a set of numbers then dividing by the total amount. ex. If your numbers are 1,2,and 3 add them to get 6 and divide by 3. You get 2.