What does the chakra at the center of India flag stand for?

The chakra in the centre of the flag is a variation of the Buddhist dharmachakra popularised by the Mauryan Emperor Ashoka. Ashoka's chakra was used for some of the first documented evidence of Buddhism, so in a sense this chakra really is the dharmachakra. The dharmachakra represents the doctrine of dharma, Buddha's stated path to enlightenment. The spokes of the chakra represent different things according to the number of spokes in the wheel; in the case of Ashoka's chakra with 24 spokes, the spokes represent the Twelve Laws of Origination and Twelve Laws of Termination. The turning of the wheel represents Buddha's sequential teachings of dharma, though in the case of the flag's chakra may more metaphorically represent progress or national strength and endurance over time.