Hi Tony. I think it has to do with tax breaks that property owners get when they use their property for certain purposes (e.g. agricultural). Rollback taxes can be assessed when the qualifying use stops. The rollback tax usually is the deferred tax plus some interest (and perhaps even penalties). I hope this helps.
Typically developers can keep property valued as agricultural with a few cows grazing on it and therefore lower taxes. When it actually starts to be develped the taxing authorities will sometimes go back x number of years and access the higher value the property would have had as commercial and access the taxes. Check out http://www.window.state.tx.us/taxinfo/proptax/tx96_295_00/ag