"Free Vote"

When a bill is being voted on and the leader of the party says you don't have to vote with the party's official view (free vote). This implies that usually you vote with your party (unsurprisingly) but what's to stop you from not voting with your party on all occasions?

You see, politicians vote in one of two ways:

1) To please their supporters. The goal of the modern politician is to stay in office for as long as possible. Holding a public office has been turned from a civic duty into a career. It's a complicated game, but basically, elected officials will try to please the voters on the issues they give a shit about.

2) To please anyone that throws them a few bucks. Lobbyists have thoroughly fucked our country over. For complicated or boring issues that voters don't care about, the vote goes to the highest bidder. It's another complicated game, but that's the gist of it.

Generally, 2 will trump 1. Reason being that there's usually enough money involved to placate bad press with a PR/spin/smear campaign.

Usually, a political party will "encourage" (read: threaten) a stance on an issue. A "free vote" means the party has no official stance on an issue, so the individual can choose 1 or 2 depending on circumstances.