Increasing the concentration increases the speed of reaction by... increasing the number of collisions between products increasing the amount of energy in the system increasing the number of collisions between reactants lowering the activation energy of the reaction

increasing the number of collisions between reactants

Explanation:

A reaction is possible due to collisions between the reactants to create products. If we increase the concentration of reactants then that means that we are increasing the molecules of the reactants which would give them more chances to collide and make products.

For instance, using a non-chemistry example, if there are few cars in a large city, the accidents there will be less because the cars will have such space that they wouldn't collide with each other. If the number of cars in the city is increased however, there is now less space and more chances for cars to collide.