What is exactly access arguments used for in hook_menu?
Access arguments is the key for an array which contains the user permission. This means that current user must have this permission to access this page. 

We can use already defined user permision or can create new permission for our menu item.

If you go to permission page (Administer >> User Management >> Permissions), you can see the list of string you might use for access arguments. 

If you want to create new permission for your menu item, you can implement hook_permission in your module.