The Add Logic button allows you to direct your survey taker to a specific question according to an answer that they provided. For example, if they were dissatisfied with the service, they can be directed to a question asking what they were dissatisfied with specifically. If they were happy with the service, they can be directed to the next question, and skip the more specific question. This allows for a truly customized experience for your audience.
NOTE: On the Survey/Form Settings step the "Show survey/form on a single page." checkbox must be unchecked in order to utilize logic with your questions.