If you don’t use all your data in one monthly period, then you get a discount on the next. That’s currently £1.25 per GB, so if you end up with 3GB of unused data at month end then your next month will be £3.75 cheaper. Plus, it’s calculated per megabyte, not rounded down to the nearest gigabyte.
Other networks have data rollover, but Smarty is the first we've seen that actually gives you money back. However, this credit back doesn't work on all Smarty plans so check the one you're interested in.