When you're placing a trade, "quantity" refers to the size of the trade you're carrying out.
The "quantity" figure is presented as a number, to two decimal places before 1 and in integers after 1. This number represents how much money you are going to use in that trade.
The "quantity" figure is a number relative to a "lot". A "lot" is 100,000 units of a currency. Therefore if you work in dollars and your quantity is 1, you would be trading $100,000.