The 'Close All Positions' feature is similar to the 'Level Profit' in that it will submit a bet or bets that once matched will give you an equal profit or loss on each selection in the market.
The difference is that if you have matched bets on more than one selection this button will allow you to submit all the required bets in a single click.
So to use the feature move the mouse pointer over the button and the 'Level Profit' column will display the potential equal profit or loss that can be locked in against each selection.
Clicking the button submits the bet or bets required to realise the profit or loss assuming that they get matched. Additionally you can right click on the button to add an 'Auto Close All' setting where you specify a number of seconds before the official start time of the market to make Betting Assistant automatically submit the bets required to green up the market.