When a market opens away from the previous close, the untraded band is a gap. Filling it means price later trades back across that band, usually measured as touching the prior close.
Gap fills are popular because the statistics sound impressive: in liquid equity indices a large share of small opening gaps fill the same day. The number is much less impressive once you separate gap size and cause. Small, newsless gaps fill often; large gaps on earnings or macro news fill rarely and can take months.
The dangerous version of this trade is fading a large gap on the assumption it must fill. There is no mechanism forcing a fill, and the losses on the ones that keep going are typically far larger than the gains on the ones that close.
Related: gap, common-gap, breakaway-gap, exhaustion-gap, gap-and-go