Hi Frankdude,
The line (put a space before StockSymbol)
Public Sub GetHistoricalStockPrices(ByVal StockSymbol As String)
recieves an argument that is a stock symbol (like "MSFT" or "DELL") - you knew that.
So in your code you do something like
Call GetHistoricalStockPrices(Range("G2"))
In cell G2 you have "Dell" or "MSFT"

The line of code 
...= "URL;http://finance.yahoo.com/q/hp?s=" & StockSymbol
Adds MSFT onto the string of the URL so 
URL;http://finance.yahoo.com/q/hp?s= MSFT
is what shows up in your bowser on the URL line.

I hope that makes sense.