A reinforcer is any consequence or stimulus that, when presented after a behavior, increases or maintains the likelihood of that behavior occurring again in the future. Reinforcers can be positive (adding a pleasant stimulusStimulus is defined as any change in the environment or...) or negative (removing an aversive stimulus).
Example
A student completes their homework, and as a result, the teacher gives them extra playtime during recess. The extra playtime serves as a reinforcer because it increases the likelihood that the student will continue completing their homework in the future.