Defines bytes (.db/.byte), 16-bit words (.dw/.word) or 32-bit integers (.di/.int).
You may also define string literals using these directives.
.data type expression, [, expression [, ... ] ]
.db expression [, expression [, ... ] ]
If you have a string, each character is treated as an individual expression, hence .dw "123" outputs six bytes of data for three characters.