iMovie for Mac: What is iMovie Theater?
iMovie Theater is a great presentation environment where you can view high-quality versions of your movies, trailers, and clips.
iMovie Theater is built to work with iCloud. If you have an iCloud account, movies you send to the Theater automatically appear on all your other devices, so you can easily watch your movies on your other computers, your iOS devices, and your Apple TV. iCloud keeps your movies private—they are tied to your iCloud account and cannot be viewed by others.