The mask tool is the next generation of the old "region filter". This lets you apply an effect inside or outside of an area and keyframe it so it moves across the screen. This can be any of EDIUS range of filters or you could just make the area outside the mask transparent.
In the clip here we have have cut out the shape of the one of our technical team, Ringo, to superimpose him on another image. Notice that you can soften or "feather" the edges of the mask either inside, outside or both.
You can also have multiple shapes in the same filter. If you have 3 people's faces you need to blur then put a mask on each one and they will all be blurred, all in the one filter.
Masks can be rectangular, spherical or a hand drawn shape complete with bezier curves. The only way to get better masking than this is to use a full-blown compositing program like Adobe After Effects.