The minimum trade profit percentage is used by the trade engine to determine the minimum price of sell orders. When a buy order has been filled, then the trade engine will attempt to sell the acquired funds with at least this percentage as profit (before GSMG fee).
Note that the trade engine has various smart mechanisms that may cause it to override this number and place the sell for even more profit (but never for significantly less).
GSMG evaluates an optimum for this value by virtually running the trading model on the price data of the last 250 days for this market. To make use of this optimised value, enable the option Automatically Follow Optimised Minimum Trade Profit Percentage.