Apple Final Cut Pro X (10.0.9) - Manual (Page 349)

Chapter 12 Keying and compositing 349
3 Select the foreground clip.
In the above example, the light stands need to be removed.
4 Click the Effects button in the toolbar.
5 In the Effects Browser, double-click the Mask effect to apply it to the selected foreground clip.
Tip: Type “mask” in the Effects Browser’s search field to quickly find the mask effect.