Write,restore, update the firmware
The most common uses for this function are either upgrades to new firmware versions or downgrades to older ones. Sometimes it is used to change or add languages. In 90% of the cases, most of the restart, freeze and no-boot errors are solved after a software update of this type.
The most common reason that phones are flashed is that they are locked to a particular cellular network. You may need to flash your phone in order to use it on a different network, this is known as a partial flash.
A full flash refers to actually changing the operating system on your phone. If you undertake this type of flash all of the information on your phone may be removed. A full flash could mean just upgrading or downgrading to a different version of the operating system on your phone, or changing to an entirely new mobile operating system. Flashing your phone may void your phone's warranty and it may render your phone useless depending on the security measures in place on your phone.