It could mean you're employed to walk around with the 'Here to Help' sign, we employed someone to simply greet customers as they walk in and tell them about Price Promise and that. I wouldn't really be put off by that though, it could be the case the vacancy is for stock replenishment on non food.
As for interview tips, just put yourself across as a fun, kind, and respectful, listen to other peoples inputs, take initiative, be full of energy and dress half decent.
The interview lasts approximately 40 mins, brief introduction to the interviewing team, a brief on the interviewing process, little group activity basically answering a few questions in pairs e.g. "What is good customer Service?", you're then called back one on one to discuss flexibility, you're then asked questions such as, "Tell me when you last helped someone?". They do try and accommodate you and match you to the most suitable roll e.g. if you're shy but have potential they will put you on shop floor if there is a vacancy for that type of roll.
All in all you have nothing to worry about, if you're genuine, want to work, want to get on, Tesco will get you moving. I started there as a temporary thing for summer now it's turning into a career :P
Hope the interview goes well today, let us know how you get on