Some datatypes, most notably the Text datatype, allow you to specify a default value. This will be applied to new records unless overridden by the user entering the data.

When you specify a default value, and OK the dialog, a dialog will appear asking which records you want the default value to apply to. If this is a new Table, and there are no existing records, the answer doesn't matter, however if you already have records in the Table, you can specify whether the new default value will be applied to new records only, to new records and records which contained the previous default value, or whether it should be applied to new records and all existing records (regardless of their existing value)