Hi,
this is just a command you enter in your home directory. Simply put, the permissions are stored in the file system itself, not in a separate file.
tin will work without this, but you'd get a complaint about insecure permissions for that file.
To explain this a bit further, these 3 digits represent the permissions for User (=you), Group (you can make up groups of users which will be allowed to use the same files, and some programs need that as well) and Others (=everyone else), in this order.
The possible values are 4 for read, 2 for write and 1 for execute permissions. These are added, so 600 means the file can be read and written only by you, but not by any other users.
Take a look at the manual page for chmod if you want to know more:
Hope I didn't confuse you too much
