Secondary reinforcers acquire their power via a history of association with primary reinforcers or other secondary reinforcers. For example, if I told you that dollars were no longer going to be used as money, then dollars would lose their power as a secondary reinforcer.
Here's an example of how a secondary reinforcer is established. Let's train a dog to sit. First we would introduce the discriminative stimulus, the word "sit." We could just say "sit" and when the dog sits, we would give it some food. The food would be the primary reinforcer. Immediately after we gave it the food we would say, "good dog." "Good dog" is our secondary reinforcer of praise. We would then repeat the above process many times. Gradually, we would give the food less often, but the dog would continue to sit when we told it to. The words "good dog" gradually became a secondary reinforcer.
Another example would be in a token economy. Many therapeutic settings use the concept of the token economy. Remember, a token is just an object that symbolizes some other thing. For example, poker chips are tokens for money. In New York City, subway tokens are pieces of metal that can be inserted into the turnstiles of the subway. Small debts can be paid off using tokens in New York because of the token's value of one subway ride. However, attempting to pay off debts elsewhere using NYC subway tokens would not be acceptable.
In a token economy, people earn tokens for making certain responses; then those tokens can be cashed in for privileges, food, or drinks. For example, residents of an adolescent halfway house may earn tokens by making their beds, being on time to meals, not fighting, and so on. Then, being able to go to the movies on the weekend may require a certain number of tokens.