An endowment of the Holy Ghost is the gift and potential we gain from the Lord through Temple ordinances and living worthy of them and living seeking them. As Joseph Smith said “I advise all to go on to perfection, and search deeper and deeper into the mysteries of Godliness. A man can do nothing for himself unless God directs him in the right way; and the Priesthood is for that purpose.” (Teachings of Presidents of the Church: Joseph Smith ch. 8 pg?)
The gifts of the Spirit come at many times throughout our life, some at birth. When you receive the Gift of the Holy Ghost by the laying on of hands you thereafter have the ability to continue to gain gifts appropriate to your needs and the needs of your stewardship.