What is the role of choke & starter in atube light ?

The choke is really a saturable reactor. It essentially has high initial voltage and then reverts to a current limiting mode.

The starter just heated up the filaments a bit to create a few electrons. The design of the rapid start lamp and ballast has eliminated the starter.

A starter is an electrical device used to lead current into the fillaments of a flourescen lamp.the starting current will creat electrons at the fillament of the fourescent lamp.