For the listed hypothetical reaction
Rate (fwd) = $k_"fwd"[X][Y]$
Rate (rev) = $k_"rev"[XY]$
By the 'Law of Mass Action' ...
Rate(fwd) = Rate(rev)
=> $k_"fwd"[X][Y]$ = $k_"rev"[XY]$
=> $k_"fwd"/k_"rev"$ = $([XY])/([X][Y])$ = Constant
=> The 'Constant' is defined as the Rxn Equilibrium Constant $(K_(eq))$ and is a relative measure of 'Extent of Reaction' or, how long it takes to move through the kinetic region of the concentration vs time trend to achieve constant concentrations of reactant and product. This is the 'Equilibrium Region' where Rate of Rxn fwd equals Rate of Rxn rev.