A Politics Futures bet is simply a wager on a political event, usually an election of some form, that will happen at a later date.
You can wager on a Politics Futures market at anytime, but the sportsbooks will adjust the odds in the build-up to the event in question based on any relevant news.
In general, an early Futures wager will offer the bettor with bigger odds, but come with greater risk (the uncertainty as to which candidates will drop out or will have their hopes of victory diminished by some unfolding news or scandal before the wager is settled).