A plunger functions by creating pressure (when you depress the plunger) in the drain line to loosen and move clogs, and by creating a vacuum (when you pull up on the plunger) to move the clogged material the opposite direction and sometimes to the surface.
A tub overflow drain is a second tub drain above the one in the tub floor. Since it is another drain designed to allow water to flow into it when the tub water level reaches a certain point, it is also air gap in the plumbing drain line. Its presence significantly reduces the plunger’s ability to create pressure and vacuum.
To prevent the tub overflow drain from creating a vacuum and pressure leak in the plumbing drain system, cover and seal the overflow with duct tape. Make sure you seal the edges and openings in the tub overflow drain.