It means after inserting the needle you draw back on the plunger to see if any blood is drawn into the syringe. If so it means you hit a vein and should reposit

It means after inserting the needle you draw back on the plunger to see if any blood is drawn into the syringe. If so it means you hit a vein and should reposition the needle because the shot is meant to go into the muscle rather than directly into a vein.