In the reaction #B + K leftrightarrow L + H#, if an additional B is added, what will be the result?
Let us write the equilibrium equation again,
According to Le Châtelier's principle, when a reactant is added to a system at equilibrium, the reaction will proceed in the direction to minimize the concentration (or pressure) of the reactant added.
The same explanation could be used if a product is added to a system at equilibrium.
Here is a full lesson regarding Le Châtelier's principle:
Chemical Equilibrium | Le Châtelier's Principle.