A genuflection, made by bending the right knee to the ground, signifies adoration, and therefore it is reserved for the Most Blessed Sacrament, as well as for the Holy Cross from the solemn adoration during the liturgical celebration on Good Friday until the beginning of the Easter Vigil. It is also made before the relics of the Passion of Christ.
The act of falling down, or prostration, was introduced in Rome when the Cæsars brought from the East the Oriental custom of worshipping the emperors in this manner as gods. “Caium Cæsarem adorari ut deum constituit cum reversus ex Syria non aliter adire ausus esset quam capite velato circumvertensque se, deinde procumbens” (Suet., Vit., ii). In 328 BC, Alexander the Great introduced into his court-etiquette some form of genuflection already in use in Persia. In the Byzantine Empire even senators were required to genuflect to the emperor. In medieval Europe, one demonstrated respect for a king or noble by going down on one knee, often remaining there until told to rise.
It is for this reason that this posture was introduced into the Roman Liturgy, in order that we may show due reverence to the King of kings and Lord of lords.