see this
link for an explanation of what the renaming mask does. I think it basically sets rules for automatically naming files (if you prefer to have files named a certain way so you can easily find it) or automatically set specific download locations by a rule you set for files you download.
Also, here, this second
link lists the features coming with downloadthemall (which is a download manager addon for firefox)
Once you get it working, it's pretty simple to use and it works well.