A gap is a jump between where a session closed and where the next one opened, with no trading in between. It happens because news, earnings, and overnight activity reprice the market while the regular session is closed - so the open simply starts at a new level, leaving a visible hole on the chart.

Why gaps form

Gaps are the market catching up to information it could not act on during regular hours. An earnings surprise, an overnight geopolitical headline, or a big move in global markets all show up as a gap at the open. The size of the gap roughly reflects how much the new information changed the market's view.

What "gap fill" means

"Filling the gap" means price trades back to the prior session's close, closing the hole. There is a real tendency for some gaps to fill, because the prior close is a meaningful reference level that attracts price. But it is a tendency, not a rule - plenty of gaps never fill, especially those driven by a genuine, lasting shift in fundamentals.

Gaps fill often enough to notice and fail often enough to ruin you if you trade it like a law.

Trading around gaps

The useful move is treating the gap and the prior close as levels to watch, not a guaranteed trade. Whether a gap fills depends on the context - the catalyst behind it, the volatility regime, and how price behaves at the open. Read the reaction; do not assume the outcome.