American exceptionalism is the idea that the United States is in some way fundamentally different from all other countries. This does not refer to obvious things such as its size or its geographic location. Instead, it refers to the ideals of the United States and to the circumstances in which the country was founded.
According to the idea of American exceptionalism, the US is unique in the fact that it has built its identity around concepts of rights and freedom, as opposed to building it around ethnic identity as in most countries. To be an American is not simply to live in a particular place or have a particular ethnicity. Instead, to be an American is to believe in ideas such as individualism and freedom.
According to this idea, the US was born on the idea of freedom and liberty and this idea was nurtured by such things as the American frontier. The US is, then, devoted to and based on freedom and individual liberty in a way that no other nation is.