Flashable.zip files are the zip files prepared accordingly and valid for flashing on an android device either via the recovery or the bootloader.
Flashable.zip files are usually used to update/upgrade some partitions or the android device as a whole.
The partitions that could be upgraded/updated or flashed using the flashable.zip files include the followings;
1- Bootloader
2- Recovery
3- Radio/Baseband
4- Kernel
5- ROM
6- System Applications amongst others.
How to Flash Flashable.zip Files
Flashable.zip files can be flashed from the custom recovery, using Terminal/Command Prompt or directly from the Hboot (HTC Androids).In some android devices like HTC, some security flags are placed and these prevent unsigned flashable.zip files from being flashed on such devices. Therefore, to flash flashable.zip files on devices as such, you need to turn off the securities by one means or another.
How to Flash Flashable.zip files Via Custom Recovery
1- Download the specific flashable.zip file for the phone using Google Search, or if you have specific link to download from. Just make sure that the flashable.zip is for the phone.
2- Put the downloaded flashable.zip file in the root folder of your SD Card or the phone's Mass Storage.
3- Reboot the phone to recovery using the right key combinations for the phone or by using the command line adb reboot recovery
4- From the recovery, wipe the cache + Delvik
5- From the recovery, choose Install ZIP then, select Choose ZIP from SD Card. Confirm the selection and wait for the process to complete and reboot the phone.
And by this, you have just flashed the flashable.zip file on the phone. You can now reboot the phone to test/observe the changes you have made on the phone.
Safety TIP:
Do always backup the whole content of your phone using a Custom Recovery before you attempt to flash any flashable.zip file on your phone.
In case anything goes wrong, you would be able to reverse all the actions.