Receptor reserve is where there are more receptors than necessary to elicit the maximal effect possible from agonist binding. This is frequently seen in muscles, where it is only possible for a certain size of contraction to be obtained. Receptor reserve is beneficial, as receptors can be lost over time. By having this reserve, it is possible to ensure that the cell can produce the maximal response to the agonist over its entire lifespan.
The impact this has on agonist binding is that there is a limit to the number of agonists that bind to elicit an effect. After a certain point, further agonist binding has no further physiological effect.