A sell-side imbalance of $642 million indicates that there is more selling pressure than buying pressure in the market. This can lead to a short-term decline in stock prices, including the SPY ETF, which tracks the S&P 500 index. The impact on SPY is relevant as it is a broad market indicator, but the importance is moderate as the imbalance can be temporary and may not have a lasting effect on the ETF's performance.