He holds a vajra symbolizing method in his right hand and bell of wisdom in his left. Their pairing thus represents the union of wisdom and method, an important state in the eventual journey to Nirvana.
The use of the bell and vajra differs according to the ritual performed or the sadhana chanted. The vajra can be used for visualization or evocation of deities; ringing the bell can be used to request protection or other actions from a deity, or it can represent the teaching of Dharma, and can also be a sound offering. During meditation on the deity Vajrasattva, the vajra is placed on the chest of the practitioner, meaning that Vajrasattva is brought to the meditator, and they become one and inseparable. Ringing the bell then represents the sound of Buddha teaching the Dharma and symbolizes the attainment of wisdom and the understanding of emptiness.
Of Related Interest:
Vajrasattva (Brass Statue)
Vajrasattva with his Consort Ghantapani (Brass Statue)
Vajrasattva and Ghantapani in Yab Yum (Tibetan Thangka Painting)