It's Zag's friend from The Big Breakfast... 

Only Joking! 

ZIG simply means 'Zipped in Groundsheet' - all Bell Tent Boutique tents are the ZIG style, meaning you can unzip and remove the groundsheet if required (great on a lovely sunny day as you can unzip the canvas from the groundsheet 

You might also see tents described as SIG (Sewn in Groundsheet) or PIG (Pegged in Groundsheet)