The bid and ask for an ETF are similar to a stock, where the bid reflects what investors are willing to pay to buy units, and the ask reflects what investors are willing to sell units for. Unlike a stock, ETFs are open ended, meaning that supply and demand is not limited, so that a large order can result in the creation or redemption of ETF units. The bid and ask will reflect the average bid and ask of the underlying portfolio, and in addition, natural liquidity between buyers and sellers of the ETF may narrow the bid-ask spread.