What is the meaning of '@' symbol in Oracle SQL?

It refers to a non-local table, the bit behind the @ is the db descriptor.

select * from question_answer@abcd where id = '45'

Means select not from the local question_answer table, but from the table on the db designated as abcd. The keyword to google for is dblink

This is the syntax for accessing a table via a database link called "abcd" See the documentation for CREATE DATABASE LINK, or to see the defined db links:

SELECT * FROM all_db_links;