What does the reaction rate say about a reaction?

It shows the change in concentration of a reactant or product per unit of time, expressed as mol / (L · s)

Explanation:

The reaction rate is the speed at which a chemical reaction proceeds.

So if the reaction rate is the speed at which a chemical reaction happens/occurs, then the reaction rate will tell us about the concertation of the chemicals. It shows the change in concentration of a reactant or product per unit of time, expressed as mol / (L · s)