Balancer is an automated portfolio manager, liquidity provider, and price sensor.
Balancer turns the concept of an index fund on its head: instead of a paying fees to portfolio managers to rebalance your portfolio, you collect fees from traders, who rebalance your portfolio by following arbitrage opportunities.
Balancer is based on an N-dimensional invariant surface which is a generalization of the constant product formula described by Vitalik Buterin and proven viable by the popular Uniswap dapp.