The Alternative Vote (AV) system is going to be offered in a referendum, possibly on the same day in early May that various elections are being held, now that the Bill has passed from the House of Lords back to the Commons for the last time.
It's a really odd voting (and vote-counting) method, involving multiple "preferences" rather than a single vote for each seat being contested. Some electors will have their "second choices" also counted in the process, while others will have theirs ignored. Third choices and beyond could also come into play, until a point is reached where the added together figures give the impression that one candidate (per seat) has received more than half the votes.
Of course, it's all an illusion, but it looks as though the voter is "more in control" by having more than a single choice.
There's a very good piece on the subject in the Wall Street Journal (Europe) by that publication's 'editorial page editor' (whatever that means) Brian M Carney, and I think is one of the best ways to get under the skin of the matter and gain a better grasp of why it is being pushed by some, but has little genuine benefit (in fact, probably the direct opposite) to the electorate. It is no surprise that hardly anywhere on Earth is this system operated!