Intermittent reinforcement is a when a reward or punishment is given on an uneven schedule, rather then every time the desired (or undesired) response is performed. When the reward or punishment is given every time, it’s called continuous reinforcement.
Think of giving candy to a baby every time it stops crying. That would be continuous reinforcement of creating a very chubby cute baby. But instead, if you occasionally gave it candy instead, the results would be quite different and would be considered intermittent reinforcement. In fact, this latter kind of conditioning has shown to be even more effective in the long run for experiments in behavior change.
Also, think of gambling and especially slot machines. They’re famous for using intermittent reinforcement to keep gamblers hooked and spending their life savings on making Las Vegas mafiosos ever richer.