@ sign means that the files have extended attributes. What you noticed was the extended attribute
com.apple.FinderInfo, which can be modified to have Finder hide the file.
ls -@, you can see the extended attributes of the file.
This is what shows up when I run
ls -dl@ Library when my Library folder is hidden:
drwx------@ 54 me staff 1836 Aug 31 11:16 Library com.apple.FinderInfo 32
and when it is showing (a setting in Show View Options in Finder on your home directory):
drwx------+ 54 me staff 1836 Aug 31 11:16 Library
com.apple.FinderInfo 32 means that the file is hidden. (32 is just the bit they chose to represent it,
1<<5. This is called a bitmask.)
You can hide and show files with
chflags hidden and
chflags nohidden, enabling and disabling that bit of