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Short version: it indicates that there are extended attributes for this file, which are not displayed by default.
You can make them visible with
ls -@l
or
xattr <filename>
The extended attributes can have several purposes, but for example can hold meta data like that the file was downloaded from a certain website, or that certain access rights are allowed or denied ... etc.
I wouldn't recommend doing so, but you can actually clear/remove all the extended attributes:
xattr -c <filename>
Or remove just one of them:
xattr -d <attribute> <filename>
