When you apply it to a clip, you'll be able to shape the "effects area" by maniuplating the corner handles.
Then, whatever effect(s) you apply to the clip will only be applied to the video within the effects area.
I give you step by step instructions for using it in my book if you're interested.
What version of Premiere Elements are you using and on what computer operating system is it running?
To what purpose do you want to use the Effect Mask feature...blurring faces, other?
In PRE11 , You can right click the clip in Expert mode and Choose >Effect mask >Apply . You will see a box on the monitor , you can resize this box .
Go to Effects in action bar , choose any effect and drag and drop on monitor in the mask box .
You will now see the effect applied on only the portion( mask box) of entire clip .
You can also Edit or remove the Effect mask using clip>right click >Effect mask >Edit/remove