Vishnu holds the conch, discus, lotus and mace in each of his four hands. Each of these items represents some element of the world for which Vishnu is considered to be responsible.
The conch is used to symbolize the "Om", the sound that is believed to be the first sound at the beginning of creation. The discus represents the chakra, or mind. Vishnu's lotus flower represents fertility and creation. In one Hindu creation story, the lotus sprang from Vishnu's naval as he considered the creation of man. The mace that Vishnu holds symbolizes mental and physical strength. The mace also represents the strength of time in the sense that nothing conquers time and, by comparison, nothing that stands against the mace can conquer it.