What is the function of a choke in tube light?

The function of choke is to provide high voltage enough for ionization to take place in a tube light and after establishment and substenance of ionization, limit the voltage across the tube. That is the reason why a tube fuses when the choke is shorted.

The purpose of the choke is to provide a very high voltage initially between the filaments (across the two ends of the tube light). Again once the gas in the tube is ionized the choke provides a low voltage. A choke is a coil of wire.
Fluorescent tubes/lamps are filled with mercury vapor. They use electric charge to excite mercury atoms in order to produce ultra violet light. A glow starter or commonly known as starter is used in the tube light circuit to provide an initial current to filaments of the tube light.

Gases present in the tube light is of such nature that once the gas ionised then it has a tendency to increase continuously accompanied by a fall in the circuit resistance . in order to limit the current to a safe value choke is used .
choke performs two function
1) providing ignition voltage
2) limiting the current