The calculator works out the volume you can afford to trade given a cash risk and a fixed stop-loss. For example, if you want to risk $200 on EUR/USD with a stop-loss at 50 pips, then the calculator will establish that you should be trading 0.40 lots / 40K.
You define the amount to risk either as a fixed cash amount, such as $100, or as a percentage of an equity figure.
You then choose the market you are trading, and a stop-loss which can be entered either as a number of pips, such as 50, or as the corresponding offset amount, such as 0.0050.
The calculator then shows you the corresponding trade size for achieving that cash risk, as both a number of lots, such as 0.40, and also a notional volume, such as 40K.