For a reaction : `X rarr Y`, what is the significance of plus and minus signs in the following expression ?
rate=`(-d[X])/(dt)=(+d[Y])/(dt)`

Explanation

As the concentration of reactant, X is decreasing with time, therefore, a[X] is negative. But we know that rate of a reaction is always positive. Hence a negative hand, concentration of product, Y, is increasing with time, therefore d[Y] is positive.
Hence a positive sign is put before it in the expression.