Surplus funds are funds in your wallet that were not acquired by the GMSG trade engine and none of your selected markets has them as quote currency.
It becomes important if you have selected a market that has this funds as base currency and you are wondering if the trade engine shouldn't be selling your surplus funds on that market.
The trade engine will not do that voluntarily, because it doesn't know at what price the funds were acquired. So if you want the trade engine to sell it for you, you have to nudge it.
The GSMG trade engine can handle your surplus in two ways:
via the market option "Surplus selling", and
via manually adding a "fake" buy order to the order history.