After you've opened the table you want to modify in Design view, click in the Description column for the field whose Description text you want to add (or change).
Type (or edit) the field description.
Click the Save button on the Standard toolbar in the main Access window to save your changes to the table.
Although your table's field names may be quite descriptive, it's good practice to add some additional information about each field for people performing data-entry tasks. A field description is useful for this purpose. Using the Description column in the table window, you can type informational phrases or reminders for each field's contents. A field's description appears in the table window's status bar when you are entering data and that field is selected.