When a child is securely attached, he/she is "confident that his parent will be available, responsive, and helpful should he encounter adverse or frightening situations. With this assurance, he feels bold in his explorations of the world. This pattern is promoted by a parent, in the early years especially by mother, being readily available, sensitive to her child's signals, and lovingly responsive when he seeks protection and/or comfort." A Secure Base: Parent-Child Attachment and Healthy Human Development by John Bowlby
After about a year of bonding to the mother (or primary caregiver), the child has internalized memory after memory of loving experiences with mother. Their tummy now has a well of love within it. With this tummy of love from bonding with mom, the child is now interested and able (with mobility) to explore the world beyond mother. The child edges out to go explore and runs back to mommy for reassurance and care. Going on little adventures exploring new things is scary and draws from their love bank, so the child quickly returns to safety, the secure base of mother. The love and care the child draws in when returning to mother for comfort, allows he/she to venture out even farther into the adventurous world of exploring.
The mother's role after the intense bonding phase (approximately the first year), becomes a secure base where the child decides when he/she needs to return and receive comfort and care. This begins the gradual process, that will unfold over years, of leaving mother to venture into the world.