"as the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of light entering the eye flickers- the star sometimes appears brighter and at some other time, fainter."

The star light has to pass through the atmosphere. Now, the atmosphere is a mixture of gas and dust particles.

 

The starlight interacts with the gas and dust which causes it to appear bright sometimes and dim sometimes.

 

Also the presence of other artificial light sources on earth and presence of clouds change the intensity of light.