Saving files to a Mac Computer
Set permissions for items on your Mac
  
  Permission settings determine who can view and alter files on the
  computer. Permissions are an important factor on OS X and other *nix
  systems and defines the way the system handles users, files and
  devices.
  
  Assign permissions to users and groups
  
  
  Select a disk, folder, or file, then choose File > Get Info.
  If the information in Sharing &amp; Permissions isn’t visible, click the disclosure triangle.
  If necessary, click the lock icon to unlock it, then enter an administrator name and password.
  Click a user or group in the Name column, then choose a privilege setting from the pop-up menu.
  
  
  Read &amp; Write: Allows a user to open the item and change it.
  Read Only: Allows a user to open the item, but not change its contents.
  Write Only: Makes a folder into a drop box. Users can copy items to the drop box, but can’t open it. Only the owner of the drop box can
  open it.
  No Access: Blocks all access to the item.
  
  


Shamelessly copied from: OS X Yosemite: Set permissions for items on your Mac

Another useful read: Understanding UNIX permissions and chmod