Increasing concentration means more particles of reactants moving around and bumping into each other!
For a reaction to occur, reactant particles (molecules, ions or atoms) must collide with each other. This is called collision theory .
When concentrations are increased, that means that there will be larger and larger number of particles which can bump into each other.
If people were particles imagine how often you would bump into someone else if you were in this crowd compared to being in a crowed that only had 1/10 or 1/100 the number of people as are shown here.
You would definitely collide with more people more often in the larger crowd!
See how concentration changes the speed of a reaction in this classic experiment.
Hope this helps!