I've come across a *.apk where permissions are needed to be set to the *.db file. But I don't know what's it meaning. Help?
It's difficult to tell for certain without a bit more context, but it sounds like the instructions are attempting to convey the Unix filesystem permissions to set on the database.
Since Android at its core runs on the Linux kernel, its filesystem maintains the same permission and access rules as any typical Unix system does. In general, there are three relevant access modes for a file:
Because of this, simplified Unix filesystem permissions are generally expressed as triplets (I'm ignoring here other flags such as the setuid bit) in order of precedence: user/group/other. Often these will be numerical since Unix uses bitmasks under the hood, where:
Permissions are thus combined by adding the bitmask values together - to give read and write (but not execute) permission to the desired target, for example, you would want the value to be 6. Putting this together, a file with a permission of 755 would be:
Putting this all together in the context of your question implies (to me) that the instructions are thus telling you to give the owner of the .db file read/write permission, the group read/write permission and other no permissions at all. I observe this by splitting up the instructions at the slash delimiter in the text.
This would be equivalent to setting the filemode to
Izzy summed up what this would mean from a functional standpoint rather well: