To genuflect means to drop to one knee, generally while also making the sign of the cross over one’s self. Traditionally, a person drops to the right knee before the Blessèd Sacrament (Jesus’ Body and Blood in the bread and wine of Holy Communion), including before the tabernacle in which the Blessed Sacrament resides. If the bishop is present, and gives an Episcopal blessing, as he/she processes out one drops to the left knee. This is a sign of acknowledgment of the bishop’s office and ministry, and is not really a genuflection.
A profound genuflection involves dropping to both knees, and then prostrating one’s self on the floor/ground. This is reserved for when the Blessed Sacrament is exposed, e.g., in a monstrance.
Genuflection of any kind is an act of personal piety and is not required. Genuflection (the word comes from the Latin for kneeling) originated as a mark of profound respect to a ruler. It was first popularized in the court of Alexander the Great (4th C., B.C.) In genuflecting before the Blessèd Sacrament a person is acknowledging the lordship of Jesus Christ.