Meaning 1
An easy out for the opposing team; the batter hits a ball that goes almost straight up in the air and comes down for an easy catch in the infield. A fly ball is to the outfield.
Sentence 1
He swung hard and had a pop-up ending the inning.
Meaning 2
He or she made a great effort, but the result was a pop-up, and the cause was lost.
Sentence 2
Ben had a strategic theory, but it only resulted in a pop-up.