It’s the transference of 2nd/3rd/4th preference votes after a candidate is either elected or drops out.
In my case I was elected at the 3rd stage. At stage 1 a Tory candidate was elected on his 1st preference votes, and his 2nd preference votes were transferred to the other Tory, but wasn’t enough to get him elected. Stage 2 saw the lowest candidate eliminated, but the 2nd preference votes transferred wasn’t enough to get anyone else elected outright. At stage 3 the Green candidate was eliminated, and the 2nd preference vote transfer was enough to elect me. The other Tory hoovered up enough 2nd preference votes (it’s usually from the LibDems) to beat the other SNP candidate.
With STV voting it is possible to be sitting in 2nd place at stage 1 (but short of the required votes to be elected) and find yourself eliminated as those around you start getting 2nd preferences from other eliminated candidates (as happened in one of the PKC wards

).
There’s probably a YouTube video somewhere that explains it better