Corrugated means molded into tight ridges and valleys. You've probably seen corrugated cardboard or corrugated metal roofs.
Materials are put into a corrugated shape to ensure elasticity and strength — cardboard boxes hold their shape because the cardboard is reinforced with a corrugated layer inside the two flat pieces. To remember corrugated, think about corduroy pants — which also have ridges and valleys. If you're lucky enough to have had ribbon candy, that's corrugated as well.