A "porthole" is a window with a steel cover that one can close over it that's designed to withstand pressure from large waves. Most portholes are round and perhaps a foot or two in diameter, though a porthole need not be round. The steel cover usually has watertight seals, and it's also designed to withstand pressure. Thus, it will keep the waves out if the glass breaks in a storm.
It is an added expense. If you feel you can do without a "window" and want to save money, book an inside cabin. If you are at all claustrophobic, you will want a porthole or other form of window, and the cost should not be an issue.