Redline and green box with session shows the session ongoing currently. It updates every second.
Case 1: If redline is New York and its box is enlightened with green then we can say Single New York session is going on.
Case 2: If red is on New York and London and both boxes are enlightened with green then the overlap of 2 sessions is going on.
With the help of economic calendar market analysis, traders can trade with the respective currency pairs at particular session going on.