Thank you for the opportunity to help you with your question!
retro-diels-alder reaction is a form of distillation process. cyclopentadiene exists as a dimer. By distilling it and keeping it cold, slows the Diels-Alder reaction with itself after the distillation. Keeping it cold doesn't completely stop the reaction, so the cyclopentadiene must be used soon as it will reverse back to its dimer.
Please let me know if you need any clarification. I'm always happy to answer your questions.