The word Soffritto literally means "sub-fried" or "under-fried" because the lower heat used to "fry" your diced aromatic vegetables (usually onions, carrots, and celery) and the quantity of olive oil is very little (2-3 tablespoons). Instead of olive oil butter can be used, or the two in combination. Soffritto is usually the first step to prepare a recipe (risotto, marinara sauce and other ragú (Bolognese) sauces, soup, and more), and the base of many common Italian dishes. The basic Southern Italian soffritto is prepared with olive oil, chopped onions (or scallions) and sometimes chopped garlic. In Northern Italy the most common soffritto is made with minced garlic, celery, carrot and onion. Nonna Iole's recipe includes Italian parsley and basil which add aromatic flavors to the mix.
While olive oil is typical of traditional Italian cuisine, some regional recipes may call for butter as the main cooking fat of a soffritto (especially in Lombardy and Piedmont, but also in other parts of Northern Italy like Veneto). Traditionally, for climatic reasons, these regions had smaller olives available. In Northern Italy the cooking fat may also include corn or seed oil.
When your soffritto softens, generally after two minutes (they say that the onion becomes "translucent"), you will add the other ingredients necessary for your recipe (rice, meat, vegetables, broth, or tomato sauce, etc.).