The Communist Manifesto was a relatively short document that was meant to be a statement of the beliefs of the Communist League, which was a multinational political movement (albeit a small one) in Europe in the 1840s. In this document, Marx and Engels lay out their vision of what the communist party was attempting to do. This was, in essence, a catechism of what the communists believed.
In the manifesto, the authors lay out their understanding of history and society. They show how history is (again, this is in their minds) a history of the struggle between classes. They show how the capitalist system that is just rising to its full strength in Europe is based on the exploitation of the workers. They then argue that capitalism will inevitably be replaced by socialism and then by communism.
Thus, The Communist Manifesto is the statement of the political beliefs of a group of European communists in the 1840s. It became one of the most influential statements of political and social beliefs in the history of the world.