What is meaning of @ in drwxr-xr-x@

From man ls:

If the file or directory has extended attributes, the permissions field printed by the -l option is followed by a '@' character.

Therefore, the file in question has extended attributes. What they are is application-dependent. I don't have a file to compare to see if the extended attributes are standard or not.

From what I observe I get the @ on files that are executable scripts (but not always), excel files (with .xls) and data files with .dat (but not always).
As far as directories go - the ones I copied from another Mac have the @, but the new ones I made recently.
I guess I just added to the confusion.

I think that's coincidence as much as anything.

Any application can assign extended attributes to a file. The Finder, for example, adds a flag that indicates the file should be quarantined (e.g. it was downloaded from the internet and hasn't yet been run). Other applications can add their own data.

Extended attributes include such things as resource forks and file types & creator codes. I was just experimenting, and noticed that if you hide a file's extension in the Finder, that adds an extended attribute. You can display the extended attribute keys using the -@ flag in ls, e.g.,

ls -@l

File types, creator codes, hidden extensions, all appear as "com.apple.FinderInfo"; resource forks appear as "com.apple.ResourceFork".