Please explain this basic code - New to VBA

Hi-

I'm just stating out with VBA assigning basic macros to buttons and such for now.
I was wondering if somebody could explain this code to me. It automatically updates an advanced filter based on the given criteria. (Code from extended office)

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Range("A5:D21").AdvancedFilter Action:=xlFilterInPlace, CriteriaRange:=Range _
("A1:C3"), Unique:=False
End Sub

Also, how do I post code so it shows in a better format?
Thanks!