When a new North American corporation is formed its founders almost always have common stock. Elsewhere similar shares of stock might be called “ordinary shares” or “voting shares.”
It is a security that gives the owner the right to share in company profits as well as vote on certain issues such as the board of directors.
Common stock owners are also entitled to “whatever is left” when a company is sold, after debt holders and preferred stock holders receive what is due to them.
In regular usage people will often refer to it just as “common.”