According to the Collision Theory of chemical kinetics:

1. Collision Frequency ($Z$): This is defined as the number of collisions per second per unit volume of the reaction mixture . When the concentration of reactants increases, the number of molecules per unit volume increases. Consequently, the probability of these molecules colliding with each other increases, leading to a direct increase in the collision frequency.
2. Activation Energy ($E_a$) and Threshold Energy: These are energy barriers characteristic of the specific chemical reaction and its mechanism. They are affected by the presence of a catalyst (which provides an alternate path with lower $E_a$) but are independent of the concentration of reactants .
3. Heat of Reaction ($\Delta H$): This is a thermodynamic state function determined by the difference in enthalpies of products and reactants. It does not change with concentration.

Therefore, increasing concentration increases the rate of reaction primarily by increasing the collision frequency.