This is to change the type of Bunsen burner flame. A closed air hole gives a bright orange dirty flame, known as a safety flame. The closed hole lets in oxygen. It is blue and less visible, and is known as a heating flame.
What Is The Function Of Bunsen Burner Air-hole?
It regulates the gas/air mixture, and thereby the temperature of the flame
The bunsen burner air hole lets in oxygen for the flame to burn on
To adjust/change the flame between luminous flame or non-luminous flame.