If you use iMovie on your Mac, you have a small update waiting for you in the Mac App Store. Today's update is small in stature but provides something every user wants — bug fixes and stability improvements. In fact, that's all it addresses according to the official changelog in the App Store.
Improves stability and resolves issues that could cause iMovie to quit unexpectedly
The update, of course, is free. If you don't have a copy of iMovie you can buy it in the App Store for $14.99.