STV is arguably the most complicated of all the electoral systems we’ll be discussing. In short, it’s like Alternative Vote for multi-member constituencies.
With STV, as a proportional system, STV would give a non-majority result to the House of Commons, which can be seen below. Widely speaking, however, the party with the most votes gets around 40% of the seats in Parliament.
The system itself is fairly difficult to explain as each candidate has a quota of votes to meet to be elected, and the excess votes, and those for eliminated candidates, get transferred to other preferences.
This CGP Grey video is the easiest way to explain it: