For a generalized reaction............
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 simply rate constants that must be measured by experiment. The condition of chemical equilibrium DOES NOT specify cessation of chemical change BUT equality of forward and reverse rates........And thus.....
"rate forward"=k_f[A][B]-="rate backwards"=k_r[C][D]....
And so on rearrangement.......
k_f/k_r="rate forwards"/"rate backwards"=([C][D])/([A][B])
And the quotient k_f/k_r is otherwise known as the "thermodynamic equilibrium constant", K_"eq", which MUST be measured for a particular reaction.
And while, I admit, so far we have been very abstract we can clearly surmise that if K_"eq" is numerically LARGE, then the products [C] and [D] are favoured at equilibrium. And likewise, if K_"eq" is numerically SMALL, then the reactants [A] and [B] are favoured at equilibrium.
And if you can digest all that (and I don't know at which level you are) with understanding, then congratulations, you now understand "chemical equilibrium" to undergraduate level. Anyway, you have to see how to use this knowledge in the questions you are likely to be asked. And the definition you must know is that "chemical equilibrium" specifies equality of forward and reverse rates of reaction.
