If I'm reading the offer correctly, it sounds like if you go shopping in Ikea between 31st October and 4th November, then you can have an Ikea restaurant bill from between 17th and 21st October, deducted from the store bill.
If you were to go shopping in Ikea within the required dates, you could eat as much as you liked in the Ikea restaurant between 31st October and 4th November for free (up to the amount you end up spending at the later date).
Here's the relevant page in the catalog: http://onlinecatalogue.ikea.com/GB/en/2012/IKEA_Catalogue/?&page_no=372-373]
.. I'm imagining someone buying £100 worth of meatballs, and then having it subtracted from their future sofa purchase :)