Text domain missing; what does this mean?

RECOMMENDED: Text domain problems in archive-unit.php. You have not included a text domain!
Line 83: <h1 class='zero-course-units'><?php _e( '0 units in the course currently. Please check back later.' ); ?></h1>
RECOMMENDED: Text domain problems in archive-unit-workbook.php. You have not included a text domain!
Line 81: <span class='unit-archive-single-module-status read-only-module'><?php _e('Read only'); ?></span>
Line 93: <h1 class='zero-course-units'><?php _e('0 units in the course currently. Please check back later.'); ?></h1>
RECOMMENDED: Text domain problems in archive-unit-grades.php. You have not included a text domain!

Where and how do I fix this?