Surplus selling is a feature that can be enabled for the trade engine. This can be done via the "Markets" option in the menu and then open the settings for a market.
Surplus is funds in your wallet that acts as the base currency for one of your selected markets, but was not acquired by the bot and is also not a quote currency in any of your (other) selected markets.
Enable surplus selling if you would like the bot to sell this surplus for you on this market. Every few hours the bot will try to place an order for approximately 3% of your surplus amount, until all of it is on order.
Optionally, you can specify a minimum sell price.