Complementary foods refer to those foods that are given in addition to breast milk or infant formula during a baby's first year. These foods provide essential nutrients and help the baby grow and develop properly. Complementary foods should be introduced along with breastfeeding, and they can include pureed fruits and vegetables, whole grains, protein-rich foods like eggs or meat, and iron-fortified infant cereals. The timing of introducing complementary foods varies from baby to baby but is usually around 6 months of age.