AV doesn’t really change how proportional our system of elections would be, but what is proportionality in politics?
Under our current system, because of the fact that each constituency is a separate election, larger parties tend to get more of the seats. That means that some voters have more influence over the country than is strictly fair. Look at the following example from the 2010 general election (votes are on the inside, seats in parliament on the outside):
No party got the same percentage of seats as they did votes.
Proportional systems try to correct this so that each voter’s has equal influence over the country.
There are several different proportional systems, each of which elects the representatives differently, but none of them is on offer in the referendum of 5th May.