Lists are the participants in your survey(s), these are QR-codes which you can have on different places.

Example 1:
You want to send a survey to 1000 addresses and you want to generate QR-codes for all these addresses, then you will need to add those 1000 addresses to a list.
When you activate a campaign, you'll get 1000 QR-codes which you can use for sending out.

Example 2:

You have five locations and you want to measure customer satisfaction. In this case you will have te make a list with five participants. When you activate the campaign you will receive five different QR-codes, which will all lead to the correct survey. You can use these QR-codes as much as you want and even print them out, there is no limit. For example, when you want to measure customer satisfaction at the reception and the waiting you'll need to add a participant for both these rooms.