A flash file (or stock ROM) is the original operating system software for your device. Flashing this file onto your phone is essentially like performing a "factory reset" on steroids—it replaces the entire software system to fix deep-rooted problems that a standard reset can't touch.

When the phone hangs on the "Nokia" screen during startup.

Key components of an SPD flash file package

It will corrupt the modem and IMEI. Always check under Settings > About Phone > Model Number, or look at the sticker under the battery cover.