Relative references change when they're copied to another cell, whereas absolute references remain the same regardless of where they're copied. For example, if you were to make a spreadsheet to show how much money would be made on different days at a cinema you would use an absolute reference to keep the price of the seating the same for each type. You would do this by placing a $ before the column number (if you wanted that to be the same) or the row letter (if you wanted that to stay the same).

https://support.office.com/en-ca/article/Password-protect-worksheet-or-workbook-elements-dbf706e0-ba22-4a08-84d8-552db16eef11