I confess to not being familiar with that term?
Free surface... it's basically a half-full tank of some liquid. Very dangerous. An empty tank or a full tank doesn't impart any unpredictable momentum vectors to the ship. A full tank of liquid behaves no differently than a full tank of grain or other dry stores.
But a half-full tank, say, a half full tank of oil or gas or water-- if there is any motion that gets the tank sloshing side to side, even just a little bit-- is bad news. It's the amount of free surface of the fluid in the tank that matters, and it is surprising how little motion it takes for many tens or even hundreds of tons to suddenly be on the wrong side of the ship at the wrong time.
Granted, we're talking about large tanks here. Small potable water tanks or fuel tanks are usually of no consequence. But large cargo-type tanks, or even fish holding tanks are something to be careful with.
There was a sea story I told here once about a shrimper up in Alaska that had capsized and only the skipper made it off alive. Free surface was a big part of his problem. He was transferring fuel between two large tanks, but also had some water in a shrimp holding tank. Shrimpers are top-heavy enough to begin with... but just a little wave got things to sloshing... and bloop... over he went in seconds.