SQL Queries - Explanation Of Od

I am wondering what are od1 and od2 used for in SQL Queries? Are they used for joining... can anyone explain their significance in the queries below... (especially the commands in red)


USE Northwind
SELECT OrderID, CustomerID
FROM Orders o
WHERE 20 < (SELECT Quantity
FROM [Order Details] od
WHERE o.OrderID = od.OrderID AND od.ProductID = 23)USE Northwind
SELECT DISTINCT ProductName, Quantity
FROM [Order Details] od1 JOIN Products p
ON od1.ProductID = p.ProductID
WHERE Quantity = (SELECT MAX(Quantity)
FROM [Order Details] od2
WHERE od1.ProductID = od2.ProductID)