Does exactly what it says in the tin. You supervise people. Noramly its a role on checkouts where there are a number of people on the tills and the supervisor will be responsible for looking after them, getting change, dealing with customer questions etc. Most of them are jumped up hitlers with a superiority complex.
Yes WoWo, especially the Supervisor who struts around in Asda, Dunstable, with a massive bunch of keys draped round her neck like the Lord Mayor of London! Look at me I'm important!
Smudge, reminds me of the Jack Dee sketch about the manager at an off-license with a huge bunch of keys on their belt. "You see these keys? These signify responsibilty".
Same goes for collegues I've seen with a pager, two mobile phones and a Leatherman Pliers/knife thing all in pouches attached to their belt.