Flashing is changing the phone's internal software, or firmware. This can be done to upgrade to a newer version which has bugs fixed, or to modify the phone's features (add ringtones, change graphics, add features). In some cases, flashing can change a phone's model. (8210 can be 8250, 3310 can be 3315). It is called "flashing" because the phone's operating system is not on a hard disk, but in a chip which is direct programmed, a "flash memory" chip, hence "flashing".