According to the Le Chatelier's Principle, if a condition of system is changed, then the system try to become equilibrium again by correcting that change.
N2(g) + 3H2(g) ⇄ 2NH3(g)
If H2(g) is added, then the concentration of H2(g) is high in the system. So the system tries to reduce it and become to equilibrium state. Hence to reduce that H2(g) the forward reaction is promoted. Then Concentration of product, NH3 gas is increased.