What does the SQL # symbol mean and how is it used?
They normally prefix temporary tables.
From the docs....
Prefix local temporary table names with single number sign (#table_name), and prefix global temporary table names with a double number sign (##table_name).
The pound sign # is used to prefix temporary tables and procedures. A single instance (#) refers to a temporary object that lives/dies with the current session while a double instance (##) is a global object.
The other answers are correct if you're dealing with SQL Server, and it's clear that you are. But since the question title just says SQL, I should mention that there are some forms of SQL such as MySQL where a pound sign is used as an alternative commenting symbol.
#This is a comment.