The association rate constant (k_on) is a measure of how quickly reactants come together to form a complex or association product in chemical reactions. It represents the rate of the forward reaction and is an essential parameter in describing the kinetics of molecular interactions, such as enzyme-substrate binding or ligand-receptor interactions. Together with the dissociation rate constant (k_off), it determines the equilibrium constant (K_d) for the association-dissociation equilibrium, providing insights into the binding affinity and kinetics of the interaction.