Q: What is fire vault and what is it's function?

Do you mean FileVault?

If so, it is a method of encrypting the data on the disk drive to prevent access to the data by others. It is of prima importance for mobile devices that are carried around with you as opposed to desktop devices that tend to remain in more secure environments.

walter from the woodlands wrote:

 

What is Fire Vault and what is it"s function?

FileVault is how you prevent data loss and data exposure, whether you're on a mobile computer or a desktop, or your computer lost or gets stolen, or your disk fails and you can't access and successfully erase its contents, or should somebody gains physical access to your computer and starts looking around, among other uses.

Here is what Apple says about FileVault:  Use FileVault to encrypt the startup disk on your Mac - Apple Support