The “little round plug thing” Lobsang refers to is called the starter. This is needed in some types of fluorescent fixtures because of the way the tubes work. Inside the tube at each end is a filament, similar to those found in incandescent light bulbs, but of a much lower wattage. These glow a dull red color and are used to vaporize a small quantity of mercury. The starter’s job is to provide current through these filaments, then cut them out to allow the voltage across the tube to ionize the mercury vapor, giving off ultraviolet light. This UV light then stimulates a phosphor mix lining the inside of the glass envelope to give off a mixture of colors to approximate white light. Inside the starter is another filament which heats a bimetallic strip, which in turn makes or breaks the connection to the tube filaments. If there is not enough current through the bulb, then this filament heats up again, repeating the process until the tube ignites. You’ll often hear the starter click several times before the tube lights. If replacing the starter doesn’t fix the problem, and the tube is good, then the next thing to look at is the ballast, which limits the amount of current through the bulb once it has ignited.