help me explain this code follow??

I have some code Store produre follow, how this code run?Explain help me.

USE [ThoiTrangOnline]
GO
/****** Object:  StoredProcedure [dbo].[sp_GroupUser_GetByTop]    Script Date: 07/23/2014 21:05:58 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[sp_GroupUser_GetByTop]
@Top nvarchar(10),
@Where nvarchar(500), 
@Order nvarchar(500)
AS
Declare @SQL as nvarchar(500) 
Select @SQL = 'SELECT top (' + @Top + ') * FROM [GroupUser]'
if len(@Top) = 0
BEGIN
Select @SQL = 'SELECT * FROM [GroupUser]'
END
if len(@Where) >0
BEGIN
Select @SQL = @SQL + ' Where ' + @Where
END
if len(@Order) >0
BEGIN
Select @SQL = @SQL + ' Order by ' + @Order
END
EXEC (@SQL)

Thanks all.