After the response the positive reinforcer is removed which weakens the frequency of that response. Trainers say we reward behaviours we want and ignore what we don't want. Classical conditioning involves simple stimulant response reactions, ie Pavlovian conditioning takes place. The dogs didn't have to do a thing for the food. The only behaviour that was positively reinforced was the expectation of being fed having received the stimulus.
Conditioning is the learned association or connection of one thing with another.
Operant' means a behaviour that operates on the environment. So when you see the word 'operant', substitute 'behaviour' to make the meaning more clear. B.F. Skinner, who spent a whole career documenting the contingencies of reinforcement, outlined the principles of 'operant conditioning'. Operant conditioning means using the concepts of positive and negative reinforcement and punishment or correction by association. Secondary reinforcers are those things which, through operant conditioning, are associated with the primary reinforcers, eg 'click', verbal praise, feedback, etc.