What is the use of the rubber stopper in the laboratory?

Answer 1

From Wikipedia:

A bung is truncated cylindrical or conical closure to seal a container, such as a bottle, tube or barrel. Unlike a lid which encloses a container from the outside without displacing the inner volume a bung is partially inserted inside the container to act as a seal. The lids for safety overpacks for 55 gallon drums sometimes may have a bung built in for access of the contents of the container. These may be referred to as fuel bungs.

A rubber stopper is sometimes called a rubber bung, and a cork stopper is called a cork. Sometimes bung stoppers used for wine bottles are referred to as "corks", even though made from another material.

Answer 2

The rubber stopper is used to seal containers, such as test tubes or flasks, to prevent the contents from spilling or evaporating. It also helps to create airtight conditions for reactions or storage of samples in the laboratory.