The checkboxes should be quite self-explanatory. The UNIX permission system makes distinctions between the owner of a file (or folder), the group the item belongs to, and everybody else. Each of these has its own set of permissions . There are again three different permissions: Read, write and execute.
You will find a similar display of the permissions in FTP apps and elsewhere, where you can change file permissions.
Whenever you change something (either in the text fields or the checkboxes, all three representations will be automatically updated.
You can also paste permissions in numerical or textual form from any source into the text fields and look at the checkboxes to see what they mean. If you enter an invalid value into one of the text fields, it will turn red until it is fixed. This also happens when the text in the field is too short or long to fit that field.
The numerical representation of the UNIX permissions is calculated like this: The first digit represents the permissions for the owner of the file, the second shows the group's permissions, the third shows the permissions for the rest.
Now, the read permission is given the value 4, write is 2 and execute is 1. All permissios that the owner/group/everybody has are added up. So if the owner of a file can read and write, the group can read only and everybody can't access the file at all, you'd go like this: Read is 4, write is 2, that makes six. Read is 4 and the group can't do anything else, so it's four. And everybody does not have any permissions, which makes zero.
That way, the numerical representation is 640.
The textual representation looks the most complicated, but is actually quite simple. It consists of ten characters, where each character stands for one permission, except for the first character, which shows whether the item that we are currently examining is a file or a folder. If it is a folder, the first character is a "d", otherwise it is a "-". Now come the other.