'Reinforcement' is anything that increases the chances that an organism with repeat a behavior. When you are teaching a behavior, in the beginning, eliciting behavior takes a lot of reinforcement. So, for example, if you want to teach you dog to come to you every time you go to the back door, you feed him a cookie when you go to the back door. After a while this behavior becomes REALLY strong. He expects a cookie. Now you can work on another principle of operant conditioning called 'extinction'. It works like this. If you stop reinforcing a behavior, the dog in this case, gets frustrated, and trys really hard to get that reinforcement to continue. There are two possible outcomes. Either A: extinction, where the behavior that was being reinforced vanishes, or B: The behavior escalates until you reinforce it, and you get a new level of behavior. (this is how bad habits get worse, and good habits get better)
So, if you want to get a really strong behavior you need both kinds of reinforcement schedules. First you start off with a fixed reinforcement schedule to build the expectation of reinforcement, then you switch to a reinforcement schedule that causes some level of frustration to elicit the extinction response, and you intentionally 'fail' extinction to get a stronger behavior.
A completely random reinforcement schedule is not nearly as effective as a trainer that is tuned into the emotions of the animal that is being trained. A really good trainer can cause just enough frustration to get a little more behavior, without causing so much that the animal just gives up.