A successful shield bash has the opponent make a bad move, i.e. their combat window turns red and they moved at extremely slow speeds. The length of this condition varies from under a second to up to 4 seconds.
Depending on your shieldbashing skill and the defence of your opponent, you may also do a slight bit of damage. This is however extremely low amounts when done to other humans, rarely crossing the 5 damage mark (Unless your bashing a unarmored foe). Other factors Factors in this the damage include your you and your opponents opponent's body strength. And , as well as what armor (, if any) , the person being shield bashed is wearing.
Shieldbashing also resets the stance your opponent is in. If your opponent is aiming upper center and he is shield bashed, he will be pushed back into center stance. There is also a slight chance that your opponent will lose a bit of focus.