What is the meaning of the N string operator ?

In the SQL code related to Greenplum Database (which is based on
PostgreSQL) generated by a query and reporting tool, a noticed a "N"
operator put before a lot of strings.
For example:

case  when column_a <> N'#' then column_a
       when column_b <> N'#' then column_b
       else NULL
end

I executed this SQL code, and I found out that it is valid.

In the PostgreSQL documentation I didn't find a description of this
operator (though it is not easy to search for).
However, I found a description of the E (escape) and B (binary) string
operators.

Can anybody explain the meaning of the N string operator ?
Is there a section in the PostgreSQL documentation that describes it ?

Thanks,
Danilo Fortunato