When Catholics enter a church, the Blessed Sacrament is present in the tabernacle. We believe the host reserved from Mass is the body, blood soul and divinity of Jesus Christ. Our Lord, therefore, continues to be truly present among us under the form of bread. Another reason to reserve the host is to provide Communion for those who are sick.
On arrival at church and before taking a seat, as a sign of adoration to the Real Presence, facing the tabernacle we genuflect on the right knee: the right knee touches the floor while the left foot remains on the floor to support us as we genuflect and stand again. People also make the sign of the Cross during the genuflection. Everyone, as they are able, is encouraged to make this sign of reverence, and parents and grandparents are asked to teach their children to genuflect well. If a person because of infirmity is unable to genuflect, then a bow towards the tabernacle is another way to honour Our Lord.
Hear more about the sacramentals of bowing and genuflection at: