It means once after opening the project. It remembers the prepared images for open projects. If you close the project and re-open it, it will do it again; if you close and reopen the browser it will do it again; but if you do something like change an object property while keeping the project open, it shouldn't do it again.
Thank you Ashley I understand now. May i ask if there will be an option to preview without that process in the future? Just like Construct 2. My project take very long time to preview now and i am afraid it will be pain if my project is bigger in future. Thank again for your great support.