Foundations
File Permissions
Also: permission bits, chmod
Rules saying who may read, write or run a file: its owner, its group, or everyone else.
In more detail
On macOS and Linux each file carries three permissions (read, write, execute) for three audiences (owner, group, others), shown as a string such as -rwxr-xr--. Setting a file too open is a common way for a secret to leak; setting it too closed is a common way for a service to break. The fix is the narrowest setting that still works.