You need to put this code in the sheet module for the worksheet in which you want to monitor changes.
Target is the range that changes and triggers the worksheet_change event. This is determined automatically by Excel. For example, if you enter "Hello" in cell A1, the Worksheet_Change event will trigger, and Target will be Range("A1").
If you copy A1 to B2:B4, then the Worksheet_Change event will trigger again, and Target will be Range("B2:B4").