The flashing of the phone means changing the firmware (Software) from any mobile and replace with the same version or any other version. Simply, flashing means changing the operating system.
You can flash your device with the Recovery mode or Flash tool. There are several sites which are providing the latest firmware files.
You can upgrade or downgrade the Android firmware with flashing. However, there are some important points that every flasher will need to follow. Flashing will fix software issues, IMEI related issues and improve device performance.
Flashing delete all your device data like photos, contacts, and apps, etc. Now download the firmware of your phone from provided links.