Understanding this VBA code

This is my first post, so hello to all!

I registered because there was a specific problem that I wanted solving. The problem was locking a cell after a specific date has been reached or passed.

There was an old thread here where a couple of users had written some VBA code.

However, because the spreadsheet and situation is different for me, the code can't be used exactly as it is.

Having investigated, I've managed to understand some parts of the code, but was wondering if anyone would be willing to enlighten me on the rest.

Option Explicit

Private Sub Workbook_Open()
    Const PW   As String = "secret"
    Dim rBookings As Range
    Dim cl     As Range
    Dim mArea  As Range
    With Sheets("Availability")
        .Unprotect PW
        Set rBookings = .Range(.Cells(7, 4), .Cells(7675, 10)).SpecialCells(xlCellTypeConstants)
        For Each cl In rBookings
            Set mArea = .Cells(cl.Row, 1).MergeArea
            If mArea.Cells(1, 1).Value <= Date Then cl.Locked = True
        Next cl
        .Protect PW
    End With
End Sub

First off, what does the following mean:

Option Explicit

Private Sub Workbook_Open()
    Const PW   As String = "secret"

And this?

Unprotect PW

Now, this is where it gets a bit hairy for me.

For Each cl In rBookings
            Set mArea = .Cells(cl.Row, 1).MergeArea
            If mArea.Cells(1, 1).Value <= Date Then cl.Locked = True
Next cl

Other than an earlier statement saying that it's a variable, I can't see any other definition of 'cl'. So how does the 'rBooking' variable interact with the 'cl' variable?

I assume the 'Set' statement is defining something for 'mArea'. But what exactly is it defining?

I can see the purpose of the 'If' statement, but not how it fulfills that purpose.

And a question not related to the actual code. The values in the cells I want to apply this code to are imported from other spreadsheets where they are the result of various formulae. If the date trigger is reached and the cell is locked, if the data in the other spreadsheets are changed, will the value remain the same in the locked cell?

My apologies for a a mammoth opening post, and the rather involved question, but I may be repeating this exercise over the next few months for work, and rather than repost the same question, I thought I'd try and understand how it works.