#K_(eq)=([C][D])/([A][B])#
Given #A+BrightleftharpoonsC+D#, there is a #"rate forward"#, #k_f[A][B]#, and a #"rate backwards"#, #k_r[C][D]#; #k_f# and #k_r# are some unspecified rate constants.
Equilibrium, by definition, explicitly specifies EQUALITY of FORWARD and REVERSE rates, and thus:
#k_f[A][B]=k_r[C][D]#;
Under these conditions, chemical change has not ceased, but there is no net macroscopic change in the given concentrations.
Further we can write, #k_f/k_r=([C][D])/([A][B])#, and this is usually specified under standard conditions.
The quotient #k_f/k_r# is better as known #K_"eq"#, the thermodynamic equilibrium constant, and it must be measured for a given reaction.