在这个SQL查询的上下文中，字母“a”和“s”是什么意思？

$rank = $db->query("
SELECT * FROM 
( SELECT a.*, @row:=@row+1 
AS `result` 
FROM users a, (SELECT @row:=0) s ORDER BY id ASC )
a 
WHERE id = '".$user_id."'
");

I found the following query on SO, and am trying to understand how it works. Nothing detailed was mentioned in the answer.

So far I understand everything except the single letters "a" and "s"

What do they mean?

a and s here are aliases - shorter names used to reference the items in your from clause, for convenience sake.