Genuflecting — the act of briefly bending one knee to the ground when crossing in front of the Blessed Sacrament — is among the most physically apparent forms of Eucharistic devotion.
"In the liturgy of the Mass we express our faith in the real presence of Christ under the species of bread and wine by, among other ways, genuflecting or bowing deeply as a sign of adoration of the Lord," according to the Catechism of the Catholic Church, No. 1378.
People who are physically unable to genuflect may bow toward the Blessed Sacrament.
This act is to take place any time the Eucharist is reserved in the tabernacle, either during or outside of Mass. The faithful also are to bow before the altar when there is no tabernacle behind it, and bow before the tabernacle when it does not contain the Blessed Sacrament.