The masking process starts by adding a dedicated Mask track above the clip or track that you want to mask. You can also choose to create a Clip Mask when you want to create a mask for a specific clip, rather than adding a mask track.

From there, you can choose between two types of masks:

You can apply only one function or filter to a Shape mask (and its corresponding Matte area). You can create new mask tracks to layer masks. You can also layer Panel masks.