O @ indica que o arquivo tem os atributos estendidos estendidos .
Você pode encontrar explicações na página de manual :
The Long Format
If the -l option is given, the following information is displayed for each file: file mode, number of links, owner name, group name, number of bytes in the file, abbreviated month, day-of-month file was last modified, hour file last modified, minute file last modified, and the pathname. In addition, for each directory whose contents are displayed, the total number of 512-byte blocks used by the files in the directory is displayed on a line by itself, immediately before the information for the files in the directory. If the file or directory has extended attributes, the permissions field printed by the -l option is followed by a '@' character. Otherwise, if the file or directory has extended security information (such as an access control list), the permissions field printed by the -l option is followed by a '+' character.