A chemical reaction is characterized by the change of reactant chemicals (or simply reactants) into product chemicals (or products). The rate at which this change takes places is called the reaction rate. Some reactions occur slowly - like the rusting of the steel body of an automobile. This is a low rate of reaction. Other reactons - like the combustion (i.e., explosion) of hydrogen gas. Hydrogen combustion is a reaction that occurs at a high rate. 

Reaction rates can be altered and controlled by changing a few variables. Some variables affecting the rate of a reaction are concentration of aqueous-state reactants, temperature at which the reaction occurs, the exposed surface area of solid-state reactants, and whether or not a catalyst is used. 
 

The rate at which a reaction takes place depends on several variables. The concentration of aqueous-state reactants is one of them.  As reactants become more concentrated, the rate of reaction increases.

The three trials depicted in the diagram have different HCl concentrations. The more heavily shaded (green) that the beaker is, the greater the concentration. Since HCl is a reactant in the reaction, one can rank the reaction rates based on the concentration.