An operant response is a behavior that is modifiable by its consequences. When behavior is modified by its consequences, the probability of that behavior occurring again may either increase (in the case of reinforcement) or decrease (in the case of punishment).
For example, speeding through a red light may lead to getting struck broadside by another vehicle. If this consequence follows such a response, then the likelihood of a person's responding in the same way under similar conditions should drop significantly.
It is also possible for temporal or topographical properties of behavior to be modified by reinforcement (in the case of response differentiation).
Once an operant response occurs, it may be "free" or available to occur again without obstacle or delay. This would be the case, for example, of someone picking up a stone from a rocky beach and skipping it across the water.
Other operants are only available for very limited periods of time and cannot be freely repeated. This would be the case, for example, of someone wishing their friend a happy birthday.
Operant behaviors occur at some base rate prior to reinforcement. This unconditioned level of responding is called the operant level.
The operant level is one of a number of ways (like the yoked control) in which we can determine whether or not changes in the occurrence of the operant response are due to the prevailing contingencies of reinforcement.