The MOC is the Market on Close imbalance. It is an order type which prints each day at 3:50pm EST. If the MOC is much larger than usual, such as over a billion as a guideline, then this imbalance can signify what the big money operators are doing (CTAs for example) and what they were not able to accumulate or distribute based on trading VWAP (volume weighted average price). 

In general, the MOC is often used by systematic traders as a way to close near the end of the day, or once a day without the severe volatility of the first and last candle. Initially, arbitrageurs poured so much volume into these MOC orders that they needed to introduce AM expiration on SPX monthly options in order to reduce market volatility caused by these MOC orders. 

However, nowadays–and with the help of AM expirations–the MOC usually has a seamless effect on the market because institutions can see projections of these moves; they will use these MOCs as an opportunity to be a counterparty and make large transactions with minimal slippage, which would ordinarily come from the price moving away from them as they used large size on that same scale.