A coil fold is a gentle bread-dough fold. Lift the middle of the dough with damp hands so that the hanging ends fold underneath, then lower it back into its container. The dough wraps around itself rather than being pulled over from one edge.
Used during bulk fermentation, it builds structure while handling a wet dough carefully. It is an alternative to a bowl stretch-and-fold, not an extra step required by every bread recipe.
Reference: Further reading.