Soffritto is the aromatic vegetable base of Italian cooking - onions, carrots, and celery finely chopped and slowly cooked in olive oil or another fat until soft and fragrant, without browning. It is the first step in many Italian sauces, braises, stews, and risottos, providing the flavor foundation the rest of the dish is built on.
The exact mix varies by dish: a Bolognese sauce typically starts with the classic onion-carrot-celery soffritto, while a Genovese pesto instead uses onion and garlic. The vegetables are cooked low and slow specifically so their natural sugars caramelize gently rather than scorch, which would turn the base bitter.
Soffritto is the direct counterpart to the French mirepoix (onion, carrot, celery) and the Cajun/Creole "holy trinity" (onion, bell pepper, celery), all serving the same role as a slow-cooked aromatic base. It should not be confused with sofrito, the Latin American and Caribbean sauce base with a near-identical name but different ingredients and use.