A swing state is basically undecided according to what the polls say and can easily go either way when election time comes. While some states are rather easy to predict who they will vote for, a swing state is the opposite. These swing states are very important in the eyes of both political parties because they can literally decide the outcome of the election.

In a U.S. election, each state is given a number of 'electoral' votes, based on it population. If a majority in that state vote for a presidential candidate, all of its votes go for that candidate in the election. A swing state is a state where the presidential support is very close between the candidates and could 'swing' to either candidate. This is opposed to a 'solid' state that will definitely go for one candidate or the other.