This app is designed to fix a very annoying bug : you have one or more unread sms, but you can't see them in any sms app, so you can't set them to read.
This app find these unreachables unread sms and flag them to read.
Please rate only if you have this bug.
*** KITKAT Users your attention please ***
Since KitKat, an app needs to be the *Default SMS App* in order to update the SMS.
Because of that, when you'll click on the reset button, Reset Unread Sms will ask to you to set it as the *Default SMS app*, if you don't it simply won't work.
That said, no worry, as soon as the reset process is done, you will be asked again to revert that change.