AUM stands for assets under management and represents the total amount of money invested in a specific ETF. When investors buy or sell their shares of an ETF, the AUM will fluctuate accordingly. When considering which ETFs to trade, the size of the AUM provides good insight. For example, funds with larger assets under management have greater liquidity and are more easily traded. In addition, larger AUMs often indirectly indicate that an ETF is of higher quality as it will likely have existed longer and come with a better track record.