It is customary to genuflect or bow at the waist when you approach the Blessed Sacrament, which is the Body and Blood of Christ veiled in the Bread and Wine. This is done as a sign of respect. This practice pays homage to the verse in Philippians that tells us that at the Name of Jesus every knee shall bow. It is done by placing the right knee on the ground while keeping your back straight and your hands either joined in prayer or making the sign of the cross. Mother Theresa in the picture above is doing a double genuflection because she is also bowing. (On a side note, Anglicans have been doing this for hundreds of years before Tim Tebow).