An affidavit is a written statement which you swear is the truth. Another name for an affidavit is a sworn statement. You sign this statement (with your name, or X if you cannot write) in front of someone called a Commissioner of Oaths. A Commissioner of Oaths can be a magistrate, postmaster, a bank manager, attorney, members of the South African Police Services (SAPS), and certain priests and social employees.
For example, when you need to prove something (like your age in order to get a pension) and you do not have any written proof that what you are saying is true, then you can put this information in an affidavit.
Usually the same information that is used for an ordinary statement will be used in an affidavit. All that happens is that an attorney or paralegal will turn the statement into an affidavit by adding some formal words at the beginning and at the end of the statement.