Default values are values that are used in a field when inserting a new row where the user did not give a specific value to that field. For fields' you can edit the default values within the property editor; for gridcolumns, default values can be set in the grid column selection dialog.
In addition to all characters you can type with your keyboard, the following function names are possible
%NOW% the actual date, time or datetime, depending on the fieldtype. Default is datetime
%NOWDATE% the actual date
%NOWTIME% the actual time
%TRUE% the driver-specific true value
%FALSE% the driver-specific false value