A default constraint enables you to define the value that will be supplied for a column whenever a user fails to enter a value. For example, in a table with a column called payterms, you can instruct your database server to enter "???" or "fill in later" if the user leaves it blank.
In database diagrams, you define a default constraint as a property of a column in your table. You define this type of constraint for a column by specifying a default value inside a table in column properties view. Be sure to specify the constraint with the correct delimiters. For example, strings must be surrounded with single quotes.