An incorporator is a person who signs the articles of incorporation for a corporation.
Back in the day, corporations had to have three individual incorporators, who had to meet a number of legal requirements, such as being residents of the state of incorporation. (Until a few years ago, Japan required seven!) Nowadays, one is the norm. The incorporator is usually inconsequential to the company they incorporate. An incorporator might be a lawyer, a secretary, a bicycle messenger, or just about any other sane adult; other corporations are also allowed to be incorporators in most places.
Besides having to sign the piece of paper that establishes the corporation, an incorporator may also (depending on local law):
So once the directors meet, the incorporator loses all of their authority, and goes back to becoming a regular person on the street.