An acetic acid solution is allowed to come to equilibrium: CH3COOH+H2O=H3O+ +CH3COO-?

When the silver ion is added a precipitate of silver acetate is formed, causing the equilibrium to shift to the right. Since acetate ion is removed from the solution and to form more acetate the equilibrium has to shift to the right thus causing the amount of acetic acid in the original solution to decrease.