What are aromatics in cooking?

Aromatics are generally herbs but can be anything that will impart an aroma such as some types of spice.

Another answer.

Aromatics are any of numerous vegetables used as the base of many stews and sauces. Some of the most common are onions, celery and carrots which make up the traditional Mirepoix of French cuisine, if you replace the carrots with red bell pepper than you would have the Trinity of Louisiana fame.