A buckle in a wheel is where the rim is out of true, i.e. part of the rim moves to the left of the right, it's normally not a problem until your rim hits the pads without you pulling on the brake. Buckles are fixed by tentioning spokes i.e if a wheel is buckled to the right the spokes near the buckle that are on the left hand side are tightened so they pull the rim over, removing the buckle.