Here is the trick that makes equilibrium predictive rather than merely descriptive. Build the mass-action expression — products over reactants, coefficients as powers — and then evaluate it with WHATEVER concentrations you happen to have, equilibrium or not. That number is the reaction quotient $Q$: $Q = \dfrac{[\mathrm{C}]^{c}}{[\mathrm{A}]^{a}[\mathrm{B}]^{b}}$, where the lower-case letters $a$, $b$ and $c$ are the balanced coefficients and the square-bracket terms are the concentrations at the moment you looked.

Same expression, two names. $Q$ is where the mixture is; $K$ is where it is going. Compare them and the direction comes out free: $Q < K$ means the mixture is short of products, so it runs FORWARD. $Q > K$ means it is carrying too much product for this temperature, so it runs REVERSE. $Q = K$ and it is already there. That is Le Chatelier's principle with a number attached — and the number is the part that survives an argument.

Le Chatelier himself paid for not having it. In the 1880s he ran nitrogen and hydrogen together under pressure looking for ammonia, an explosion in his laboratory nearly killed an assistant, and he gave the work up — later calling it the greatest blunder of his scientific career. The explosion was air left in the apparatus, not the equilibrium. Fritz Haber picked the reaction up two decades later, and half the nitrogen in your body has been through his process.