A market gap is the difference between the closing price of one period and the opening price of the next period. They are created between trading sessions, such as during the night or over the weekend in pre/after market hours. Market can gap up or gap down for several reasons such as earning releases, fundamental changes to a company or political turmoil; news that are released outside trading hours. More often than not, gaps are retested and closed before moving higher. From my own experience, gaps greater than 2.5% are less likely to close. However, it's wise not to chase gaps and without some sort of confirmation from other technical indicators. Look for increase volume and a follow up in the direction before placing your trade.