Complex sentence: The "51st state", in American political discourse, is a phrase that refers to areas or locales that are--seriously or facetiously--considered candidates for U.S. statehood, joining the 50 states that already make up the United States of America.
Split into simpler sentences: The "51st state", in American political discourse, is a phrase that refers to areas or locales that are either seriously or facetiously considered candidates for addition to the 50. U.S. states that already make up the United States of America.