Question

What is the meaning of negative sign in average rate of the reaction?

Answer 1

Negative rates are conventionally used to signify the consumption of species.

Rates are not negative in an absolute sense.

Example:

If the rate of reaction is given as:

$-\frac{d\left[A\right]}{dt}=+\frac{d\left[b\right]}{dt}$ here the negative sign shows that the concentration of species A reduces during the course of reaction.