A Squib may occur when a manufacturing defect causes a round to be loaded with an insufficient amount of powder, or no power charge at all. When fired, the bullet may not have enough energy to leave the barrel causing an obstruction. The next round fired may cause significant damage to the firearm.
If you suspect that you have had a Squib Load, STOP SHOOTING IMMEDIATELY. The next round could produce an explosion if the bullet has obstructed the barrel. Field strip the firearm and check the barrel for obstructions. If the barrel is obstructed, use a wooden or brass dowel rod to remove the obstruction. Once the barrel is clear of obstructions you can reassemble and continue putting rounds downrange.