There are two types of reinforcement, positive and negative. Positive in this use of the word does not mean good and conversely negative does not mean bad. Instead positive means that when a particular behavior is exhibited something is added to the process. Therefore, negative reinforcement means that something is subtracted after the behavior has been exhibited.
An example of positive: If you give your dog a treat every time you ask her to sit, you are POSITIVELY reinforcing the behavior (because you are adding something to the process)…this example happens to be good for the dog. On the other hand, if your child forgot to do the dishes and you decided to yell at them, that would also be defined as positive reinforcement (because you are adding the yelling to the situation).
An example of negative: If you take away your child’s computer game for being bad, that is negative reinforcement (because you are subtracting something from the equation). In addition, if you are always telling your child to clean their room and they finally do that…you can stop telling them to clean (which will make them happy) and that is also negative reinforcement.