What does double number sign ## mean in SQL?
## surrounds a query parameter in the Stack Exchange Data Explorer (SEDE).
The portion after the ? is the displayed default parameter value.
From the SEDE Tutorial written by Monica Cellio:

Note: the particular syntax used for parameters is specific to SEDE. Everything we've said so far about SQL is true for all flavors of SQL; this is different.
As shown in this query, you refer to a parameter by surrounding its name with doubled pound signs, such as ##MinScore##. Optionally you can specify the data type (int (a whole number), float (a number with a decimal value, like 2.5), or string): ##MinScore:int##. If you specify a type then SEDE will validate values against that type, so that if you're expecting a number for score and somebody types &quot;unicorn&quot; the query won't run. If you don't specify a type, your query may receive unexpected inputs.


## has no meaning in standard SQL. Though not the case here, it is used in Transact-SQL as a prefix to identify a global temporary object:

Rules for regular identifiers
An identifier that starts with a number sign denotes a temporary table or procedure. An identifier that starts with double number signs (##) denotes a global temporary object. Although the number sign or double number sign characters can be used to begin the names of other types of objects, we do not recommend this practice.