For a less "initiated" guy, what's that means?
For, if I decide not to grant Full Disk Access to "bash", what will happen?
When using the Bourne Again SHell (bash) in Terminal, you can access all of the locations on the drives.
If you receive the original message out of the blue, and you weren't using Terminal, then some process that was running attempted to run some shell command in Unix that needs access to some file location on the startup drive.
Might be reasonable to figure out what wants that access before granting it, but it is likely harmless.
Determining exactly what it is may prove extremely difficult.