you mean your brake arms are getting knocked side to side as the wheel spins? Basically if you can't brake well and have the pads not rub then the rim is out of whack and needs truing. If the rim is knocked out of whack because it is bent there's a limited amount of truing you can do without overtightening some spokes or having some spokes too loose so you'll have to live with a bit of wobble that comes from the metal being bent. A common way that can happen is a front tire put in a bike rack and it falls over bending the rim.