Film and Paper Stop. You can cheap out and use water but the purpose of stop is to 'stop' the developing process and prep the paper for the fixing process. Using Stop will prolong the life of your fix and makeyour prints or films developed more accurately.
Here is what Ilford have to say:
ILFORD ILFOSTOP is a low odour, citric acid stop bath which stops further development of paper or film. It also prolongs the life of the fixer bath, maintaining the activity of your fixer solution, by neutralising the developer.
We recommend that all black & white films and darkroom papers are rinsed in ILFOSTOP after development, especially when dish/tray processing paper or deep tank processing film.
ILFOSTOP is available in 500ml bottles of concentrate, which makes 10 litres of working strength solution.