When you are looking at psychosocial psychology you are looking at the theory developed by Erick Erickson that looks at the eight stages of human development across the lifespan. Instead of looking purely at how children develop cognitively this theory also focuses on how culture affects development.
When you are looking specifically at infant development you are looking at the first stage which is called Trust versus Mistrust. This stage starts at birth and continues until age one. Within this stage the child develops a sense of trust and security based on the actions of their caregiver.
Trust is developed when the caregiver is responsive to fulfilling the infant’s needs (food, shelter, comfort, etc.) When this occurs the infant will gain confidence and feel secure, even if they are in new situations. For example when a baby is comfortable around strangers they have developed a sense of trust in this developmental stage. In turn if the caregiver does not fulfil the child’s needs the infant will have a general sense of anxiety and you will see insecurity in the sense of crying and fear of being away from (or in some cases with) the caregiver. This will affect the child’s ability to feel secure and confident in trying new things later in life.