Owners of the Samsung Galaxy S5 on the Sprint network should be receiving an OS update starting today. The OTA, the first since February, adds Lollipop's factory reset protection feature. The feature is supposed to deter theft and give you recourse in case it does happen. It debuted with 5.0 Lollipop, but Samsung's customizations to the OS originally left it out.
If anyone tries to factory reset your device when the protection is enabled, the device owner's Google account credentials are required to carry out the job. Thieves will often do this to stop Google's tracking tools from locating the phone. Of course, the phone also gets more useful to someone else when the original user's junk is wiped, too. You will need a secure lock screen to use the feature, despite the fact those two things don't seem especially related.
Sprint announced the update, version number G900PVPU2BOD3, on May 5th. If you haven't been notified just yet, head to your settings, then the system tab, and finally the update software panel. This will force a check for the OTA.