This is actually a SQL question. This forum is for SQL*Developer-specific questions - not for general SQL/PL/SQL questions which belong in the SQL and PL/SQL forum under the Oracle Database heading.
But we're here and the question has an easy answer. The "@" indicates that a database link needs to be used to access the table which (probably) exists in another database. You can read about database links in the documentation.
In addition to Riedelme:
common documentation, fully searcheable, is here:
latest release (check the new features):