what

It has various meanings. Most common would be someone or something which sets something in motion. For instance the person who says "go" or fires a pistol to start a race or the person who signals an engineer to move his train out of the station. It can also mean the first course of a meal. Another example would be 20 people signed up to be in the race but only 18 actually raced. The 18 would be starters. The other two would be non-starters.