This means that $200 is the maximum value you'll get if your phone is usually valued at less than $200. If it's above that, you'll get the standard trade-in value.
So, for example, if your phone is worth $100, you'll get $200. If it's worth $123, you'll still get $200 since that's the maximum. The deal is valid Oct. 5 through 8.
Of course, your phone's value always depends on the condition it's in, the color of the phone, the storage capacity, and the carrier.
Through Best Buy, a 32GB iPhone 5 on Verizon's network in good condition is worth $225, while Walmart values it at $150. So in that case, Walmart's deal wouldn't benefit you since the most you can get is $200.
Basically, you'll want to check your specific phone model at a few different websites before jumping on Walmart's deal. If you have an iPhone 4 or 4s, chances are you won't be able to get $200 for it anywhere else, so Walmart's new promotion would be a great deal.
If you do opt for the trade in, you can put that money toward an iPhone 6 on contract with AT&T, Verizon, or Sprint. Walmart sells the iPhone 6 for $179, but if you don't feel like purchasing a new phone you can still get the value for your old phone when you trade it in.