There are many different people in this world. Some are homosexuals and some are heterosexuals. There are also transgender and some people do not like to label themselves based on their sexual identity or sexual preference. Therefore, no one can really determine if everyone in the world is really a homosexual and that they're just in a closet.
Even your good friends may be homosexuals or heterosexual but it is hard to determine what their sexual orientation is. It is really nobody’s business. To be labeled as closeted means that someone has not openly expressed who they are based usually on their sexual orientation and so they are considered to be in the closet. When the time is right, that person will come out of the closet.