Halving is an anti-inflation mechanism implemented, in the Bitcoin network. As the name suggests a reduction in the supply of new coins is cut by half (50%).
This causes BTC inflation to fall cyclically.
It takes place whenever 210,000 blocks have been mined which is roughly around once every 4 years.
Current mining rewards are 6.25 bitcoins.
Bitcoin: A Peer-to-Peer Electronic Cash System