What does "No" mean???

Dog do not understand the concept of "No".  It really has so many meanings, for example it may mean stop, don't move, don't bark, get out of the trash, stop jumping, stop chasing. And many more that will come to mind for you.  Dogs do not speak english and they are unable to differentiate meanings. When "No" is often used before punishment dogs can actually come to fear the word. It is much better to tell the dog what you want him to do.  A cue like "leave it" "sit" "come" gives the dog a clear direction of what you want. It is not a good idea to correct your dog whan they are trying to learn. You want to encourage your dog to think and try new things.  Corrections will just shut them down and cause them to not want to try.

Think about this... You are learning something new, a new language.  The instructor asks you to repeat after her.  If everytime you mispronounced the word the instructor yelled "NO!" and hit you.  How many times would you try to say the word correctly?