a blanket (or toy) that a child carries around in order to reduce anxiety anything that an adult person uses to reduce their anxiety
A baby's security blanket is a piece of cloth or clothing which the baby holds and chews in order to feel comforted
If you refer to something as a security blanket, you mean that it provides someone with a feeling of safety and comfort when they are in a situation that worries them or makes them feel nervous. Alan sings with shy intensity, hiding behind the security blanket of his guitar