What does it mean by displacement of air?

I recently did a experiment that involved measuring the volume of gas in a measuring cylinder in a water bath. One of the error of this experiment is that air is displaced by the bung as we had to put the CaC03 powder in the conical flask to react with the acid. By the displacement of air, does it mean air from the surrounding has entered the conical flask? And by the bung, does it mean this is caused by the removal of the bung? Thanks.

To displace something means to take its place.

Displacement of air means to put something into a place or location that was previously occupied by air, forcing the air to go somewhere else, or the pressure to change.