Except for columns that are assigned user-defined datatypes, each column has an explicit NULL or NOT NULL entry. You do not need to specify NOT NULL in the table definition, because it is the default.
The sample table design specifies NOT NULL explicitly, for readability.
The NOT NULL default means that an entry is required for that column, for example, for the two name columns in this table. The other data is meaningless without the names. In addition, the gender column must be NOT NULL because you cannot use NULL with bit columns.
If a column is designated NULL and a default is bound to it, the default value, rather than NULL, is entered when no other value is given on input. If a column is designated NULL and a rule is bound to it that does not specify NULL, the column definition overrides the rule when no value is entered for the column. Columns can have both defaults and rules.