When to Genuflect

"According to Msgr. Peter J. Elliot in Ceremonies of the Modern Roman Rite (Ignatius Press), genuflection on one knee is reserved for: 1) Our Lord present in the Eucharist on the altar, in the tabernacle, monstrance or pyx; 2) the cross during its veneration on Good Friday until the beginning of the Easter Vigil; 3) a relic of the true cross exposed for veneration."

Wow! The monstrance and the pyx! Do you know what they are?