Difference between double? and double variable

@ /api/reference/robot/modifypositionasync

a double? variable is used.

What is the difference between double? and regular double variable?

Can a double? variable take null value?

Is this expression correct?

double? stopLoss = null;

double? takeProfit = null;

ModifyPositionAsync (position, stopLoss, takeProfit);

Thanks in advance.

Best regards.

public TradeOperation ModifyPositionAsync(Position position, double? stopLoss, double? takeProfit, [optional] Action callback)

var position = Positions.Find("myLabel", Symbol, TradeType.Buy);
if (position != null)
{
    double? stopLoss = Symbol.Ask- 10*Symbol.PipSize;
    double? takeProfit = Symbol.Ask + 10 * Symbol.PipSize;
    ModifyPositionAsync(position, stopLoss,  takeProfit);
}