It means that your wheel probably have a bit of a side to side wobble going around the circumference.
Spin your wheel and observe the gap b/n one of your brake pad and the rim. If the spacing varies during the revolution, it's a sure sign that the wheel is out of true. Also, look at the outer circumference from the side and check if it goes up and down as the wheel turns. If not, then the wheel is also out of round. These can all explain your present problem.