The theory places a strong emphasis on the biological basis of attachment behaviours, such as a child's desire to be close to their caregiver in order to create a ‘secure base’.
Babies and young children need to have a trusted person. This person becomes their ‘secure base’ and gives them the confidence to play and explore.
So, when your toddler moves away from you but looks back and returns to you frequently, they are using you as their ‘secure base’.
Your toddler will be motivated to stay close to you when playing, but knowing you are nearby gives them the confidence to explore their surroundings.
This then creates a safe place for your toddler to come back to while encouraging them to branch out, meet new people and trust others. Their attachment to you gives them the confidence to explore the world.
“Intimate attachments to other human beings are the hub around which a person's life revolves, not only when he is an infant or a toddler or a school child but through his adolescence and his years of maturity into old age.”
John Bowlby[1]
Reference:
[1] Bowlby, J. (1969) Attachment and loss. New York: Basic Books.