What does 'ByVal' mean in this VBScript

Hi,
I have started learning a little ASP along with VBScript. I have come across one little thing in some code my book uses that does not give any explanation.

Please find below part of a class object definition thing that gives a property to a given object.

What I do not understand is what the 'ByVal' thing does in the bracket. I understand that iSpeedin is being declared but what the heck is this 'ByVal' thing and what does it do?


public Property Let CurrentSpeed(ByVal iSpeedIn)
internal_speed = iSpeedIn
End Property

Sorry if this is very simple for some people but hey I am learning :))

Thanks
Obie