A more thorough explanation:

A biological parent is a person who provides either the egg or the sperm to form the zygote that grows into an embryo. In ordinary usage, the term denotes more than responsibility for conception and birth. The term commonly includes either the natural father or the natural mother of a child, the adoptive father or adoptive mother of a child, a child's putative blood parent who has expressly acknowledged paternity, and an individual or agency whose status as guardian has been established by judicial decree.

For example, if a child is born to a woman and a man who are married, they are both considered biological parents. If a child is adopted, the adoptive parents become the biological parents. If a man acknowledges paternity of a child, he becomes the biological father.