What is the meaning of labayk?

Two author's I have been reading use this term. In Always Bell by Winifred Stegar she states that Labayk "is an all-encompassing Word of praise and gratitude and submission to the divine will." Malcolm X in his autobiography translates the term as meaning "Here I Come, O Lord." Both used the term in context of discussing the Hajj (pilgrimage) to Mecca.