Question

What do you understand by (1) Common ion effect (2) Buffer solution.

Answer 1

The reduction in the degree of dissociation of a salt by the addition of another salt which contains a n ion common to the salt is common ion effect.

a solution which maintains its ph value fairly constant even upon the addition of small amount of acid or base is called a buffer.
example : sodium acetate + acetic acid

Answer 2

common ion effect is used to describe  the effect on an equilibrium  involving a substance  that adds an ion that is part of equilibrium 
buffer solution is an aqueous solution consisting of a mixture  of a weak acid and its conjugate base or vice versa