Skinner showed how positive reinforcement worked by placing a hungry rat in his Skinner box. The box contained a lever on the side and as the rat moved about the box it would accidentally knock the lever. Immediately it did so a food pellet would drop into a container next to the lever.
The rats quickly learned to go straight to the lever after a few times of being put in the box. The consequence of receiving food if they pressed the lever ensured that they would repeat the action again and again.
Positive reinforcement strengthens a behavior by providing a consequence an individual finds rewarding. For example, if your teacher gives you £5 each time you complete your homework (i.e. a reward) you will be more likely to repeat this behavior in the future, thus strengthening the behavior of completing your homework.