Guardian of a minor (below 18 years of age) means any person who has responsibilities as well as certain rights that come with responsibilities, to take care of affairs and of welfare of the minor child.
Section 6 in The Hindu Minority and Guardianship Act, 1956
Natural guardians of a Hindu minor.—The natural guardian of a Hindu minor, in respect of the minor’s person as well as in respect of the minor’s property (excluding his or her undivided interest in joint family property), are—
(a) in the case of a boy or an unmarried girl—the father, and after him, the mother: provided that the custody of a minor who has not completed the age of five years shall ordinarily be with the mother;
For other clauses (b)-(c), read at webpage given above.
Though a father is considered natural guardian of a minor child, as of today for all practical purposes the guardian and parental or non-parental custodian of a child is decided by courts based solely on criteria of "welfare of child" and not as per statute above. Read Chapter on Landmark Judgments for judgments pertaining to this concept.