It is the expression, which shows how the reaction rate is related to concentration

In some reactions, the rate is apparently independent of the reactant concentration. The rates of these zero-order reactions do not vary with increasing nor decreasing reactants concentrations. This means that the rate of thereaction is equal to the rate constant, k , of that reaction.

Reaction that are auto catalytic can have their rate increase with time i.e. as the product is formes the product start to work as catalyst and increase the reaction.

$\frac{-1}{3}\frac{d\left[H_2\right]}{t} =\frac{-d\left[N_2\right]}{t} =\frac{1}{2}\frac{d\left[N{H}_3\right]}{t}$