What is the + at the end of Unix permissions in a long listing format?
Here's an example
drwxr-xr-x+ 6 reza reza 204 Mar 5 20:23 .ssh/
Although I am the owner of this directory, I cannot modify it without using sudo!
Why is that? how can I remove '+' here (assuming that is the problem)?
The + is an indicator that there is an extended file ACL applied to that file.
On Linux you want getfacl(1) to read that, and my guess is that the ACL is what blocks your access to modify the directory.
On MacOS you want chmod(1), which has an extended command-line language to get and set ACL entries on files.