That is a thurible. It is a metal censer held by a chain. Incense is burned in it for venerating holy objects and people during Catholic worship. This would include the altar, relics, the crucifix, the offerings at Mass, the Blessed Sacrament itself, the Gospels, the clerics, the faithful and the coffin at a Requiem Mass. The higher the dignity of the person or object, the more swings from the thurible it receives. The incense is kept in a vessel called the boat before it is placed in the thurible.