Cycle mode means that you can loop a selected part of your project over and over again. This is very useful when working on a part of your project, and need to hear it played repeatedly while you are applying your masterful tricks and magic to it!
Activate cycle mode by clicking the Cycle icon (1). Then, click and drag the endings of the cycle marker (1) to resize and select a specific part of your project. Now, when pressing play, only this selected part of the project will play and loop over and over.
(1) Cycle icon and cycle marker