From BR Bullpen

A pop-up or pop fly is a fly ball that is hit with an almost vertical trajectory. A pop-up will usually land in the infield or in foul territory not far from home plate.

Pop-ups are very easy to field, except in very windy conditions or when there is confusion among a team's fielders. Because pop-ups are considered to be virtually automatic outs, baseball's most complex rule, the infield fly rule was devised to prevent teams on defense from benefitting unduly from pop-ups hit by their opponents.