Food halls using coupons instead of cash is something we're encountered a few times in south east Asia but which we'd never seen at home in Australia. It was rather confusing the first time but its simple once you know how it works. We thought we'd explain it so you can avoid the awkward first time confusion.
Somewhere near where you walk in will be a little booth with a bored person trapped inside. You hand over money and they give you a book of coupons with various monetary amounts written on them. For example, 50 baht might get you a 20 baht coupon, two 10s and two 5s. Try to only hand over an amount you might reasonably spend. That avoids creating a scenario where you hand over a thousand Baht while only wanting a hundred Baht in coupons then realizing how completely inadequate your Thai is while they ask you whether you really want one thousand baht worth of coupons.
When you order your food you hand over your coupon book and they will tear out the coupons they need and hand the rest back to you. You then go sit down and they will either run your food out to you or get your attention to come get your food when it is ready.
I can only guess as to the reasons why food halls would introduce this system. Not having to deal with cash would certainly be handy for the food vendors. Presumably they just hand over their collected coupons at the end of the day and the money appears in their bank account. Sounds like a nice convenience.
Some places will happily refund your unused coupons. Some places will not. Generally, the amounts of money involved are not huge so don't stress too much about unused coupons. Worst case, go and get some dessert. You know, just to avoid wasting coupons.