The equilibrium means that the forward reaction and the reverse reation happen at the same rate.
Forward reaction Cl2(g) + Br2 (g) ----> 2BrCl(g)
Reverse reaction 2BrCl(g) ----> Cl2(g) + Br2(g).
When one of the reactants in an equilibrium rection is removed, the equation results out of equilibrium, and the products will increase their reaction to restart the equilibrium producing more of the reactants species.
That is named by telling that the equilibrium is displaced to the left (side of the reactants).