The phrase "brain mass" simply means some kind of growth in the brain that should not be there. A growth, more commonly called a tumor, can be either malignant, benign, or something in between. That being said, even benign tumors of the brain can cause serious problems, and can even lead to death. Why? A benign expanding mass in the brain can -- depending on the location -- eventually press upon vital structures (such as the area that controls breathing) and cause those essential parts of the brain to stop working properly.
Posted by curious_george?