How Stuff and For Xml Path work in SQL Server

Question

Table is:

Id	Firstname
4	abc
4	def
4	ghi
4	jkl
4	mno

Required output:

Id	Name
4	abc,def,ghi,jkl,mno

Query:

SELECT ID, 
    Name = STUFF(
                 (SELECT ',' + firstname FROM temp FOR XML PATH ('')), 1, 1, ''
               ) 
FROM temp GROUP BY ID

This query is working but I want to know the reason how it is working or if there is any other  way to get this done