Why does a more concentrated reactant means a faster reaction rate?
Increased chance of collisions due to more reactant molecules/ions/atoms per unit volume of reaction mixture...
Higher concentration means that there are more atoms/molecules/ions or whatever the reacting species happens to be per unit volume. This makes the chances of collisions between atoms/molecules/ions are more likely, and therefore the reaction proceeds more quickly.
However, please note that this is only the case in the absence of any other changes.
A more concentrated reactant, coupled with a lower temperature, might not result in a higher reaction rate, as one change would nullify the other.