This association of an object with oxygen is called oxidation.
In order to prevent color change, the surface of the peeled apple should not be exposed to oxygen. A common household practice is to soak apples in salt water. Once the apples are soaked in salt water, a film of salt water forms on the surface of the apples, which prevents the enzymes from binding to oxygen and prevents the color from changing.
In addition to apples, peaches, pears, and bananas will also oxidize and change color if left unpeeled. However, bananas cannot be soaked in salt water because they will not taste good. In this case, it is better to add the juice of a lemon. This is because the vitamin C contained in lemon has the function of preventing oxidation.