It is a sign of humble respect for the presence of God.

They should genuflect to the host (that is the bread made into the body of Christ). They're genuflecting to show respect to the body of Christ.

Traditionally, Catholics bowed their heads whenever the name of Jesus was spoken, and genuflected at the Incarnatus ("... and was made man") during the Creed. Some still do. It is still appropriate. Current missalettes contain the rubric (instruction) that people are to bow at this moment during the recitation of the Creed. Genuflection is an act of reverence consisting of falling onto (usually) one knee. It originated as a posture of respect of a knight before a king. Today the term is used mostly in the Latin rite of the Catholic Church. Catholics who come into the presence of the Eucharist (generally stored in the tabernacle) are expected to genuflect on the right knee as a sign of devotion. If the Eucharist is exposed in a monstrance, many Catholics genuflect on both knees. Traditionally, a priest or acolyte passing in front of the tabernacle during the Mass would genuflect each time he passed; however, recent instructions from the Vatican have indicated that it suffices to genuflect once at the beginning and once at the end of Mass. On Good Friday and Holy Saturday, Catholic Churches do not contain the Eucharist. However, there is a custom whereby the faithful genuflect to the crucifix on these days, once it has been revealed in the Good Friday service. http://en.wikipedia.org/wiki/Genuflect

the host is the eucharist which is the body of christ christ being God is the presence of God since he is fully god and fully man.

It is honor of the Eucharist which is present in the Tabernacle.

Genuflecting is a gesture of honor to Jesus and to His Real Presence in the Blessed Sacrament in the tabernacle. A candle lamp hangs near the tabernacle to signify that Christ is present inside it. You don't genuflect, at Good Friday services, because the tabernacle is empty. The door is opened and the candle lamp is extinguished, as if in mourning. You also don't genuflect when re-entering the pew right after Holy Communion, either, because the Body of Christ is now actually in you.