Questions about existing code

The code below was copied from one of John Walkenbach's CDs. It's working fine, but I have a few questions about it.

1. What does the Option Explicit do? More to the point, what does the Public Const line do? My file has nothing to do with elephants. I know I can modify it, but what will happen if I delete that line?

2. I want to close the Assistant if the workbook from which it was launched is closed. And I want to hide it if the workbook is no longer the active workbook (and then come back if the workbook becomes active again). In other words, I want the help file that is launched by this code to be context sensitive to the workbook that launched it. How do I do that?

3. Can I change the color of the help ballon from the default yellow to something else?

Thank you in advance for your consideration. Here is the code:

Option Explicit

Public Const APPNAME As String = "Elephants R Us"
Dim Topic
Dim HelpSheet
_________________________________________________________

Sub ShowHelp()
Set HelpSheet = ThisWorkbook.Worksheets("HelpSheet")
Application.Assistant.On = True
Topic = 1
With Assistant.NewBalloon
.Heading = "Help Topic " & Topic & ": " & vbCrLf & HelpSheet.Cells(Topic, 1)
.Text = HelpSheet.Cells(Topic, 2)

.Button = msoButtonSetNextClose
.BalloonType = msoBalloonTypeButtons
.Mode = msoModeModeless
.Callback = "ProcessRequest"
.Show
End With

End Sub
_________________________________________________________

Sub ProcessRequest(bln As Balloon, lbtn As Long, lPriv As Long)
Dim NumTopics As Integer

NumTopics = Application.WorksheetFunction.CountA(HelpSheet.Range("A:A"))
Assistant.Animation = msoAnimationCharacterSuccessMajor
Select Case lbtn
Case msoBalloonButtonBack
If Topic <> 1 Then Topic = Topic - 1
Case msoBalloonButtonNext
If Topic <> NumTopics Then Topic = Topic + 1
Case msoBalloonButtonClose
bln.Close
Exit Sub
End Select
With bln
.Close
Select Case Topic
Case 1: .Button = msoButtonSetNextClose
Case NumTopics: .Button = msoButtonSetBackClose
Case Else: .Button = msoButtonSetBackNextClose
End Select
.Heading = "Help Topic " & Topic & ": " & vbCrLf & HelpSheet.Cells(Topic, 1)
.Text = HelpSheet.Cells(Topic, 2)
.Show
End With
End Sub