Trying to learn VBA, using the book

Excel 2003 VBA

On pg. 20. Code:

First line:

Function InvoiceAmount(ByVal Product As String, ...)

What is ByVal? What is As String? Why this structure?

Second line:

Price=WorksheetFunction.VLookup(Product,Table,3)

Does this mean that Product has already been defined as a name range?
Does this mean that Table is a defined name range?

you can't learn a language just with a book.
Install VB6, VBA and all the VB documentation (help or even better MSDN). write some code. if you need help, highlight the word (for example ByVal) and then press the help (F1) button.
Marco