Capers are actually the immature, dark green flower buds of the caper bush, an ancient perennial native to the Mediterranean and some parts of Asia. Capers are most often preserved by pickling them in a brine of vinegar, salt, or wine. In addition, they can be dry brined in salt.
If you ever made chicken piccata you are already familiar with capers. Their taste evokes the flavors of tangy lemons freshly picked on a bright blue Summer day, combined with the brininess of green olives, and earthy flavor notes.
In the past, capers were primarily used in dishes originating in and around the Mediterranean; especially in seafood dishes and pasta sauces (puttanesca sauce). However, as the world grew smaller and capers were introduced to other cultures, they expanded into other dishes such as, chicken, stews, combined with spreads; even added to dressings, and compound butters.
One way I like to use them is to fry them up in a bit oil until crispy and add them as a garnish to things like salads, or as a topping to a good steak or piece of fish.
You can find them in the aisle where condiments are located, usually close to the pickles. In the United State we consider them a condiment; however, in most Eastern countries they are thought of as just another ingredient.
They can be used raw in things like salad dressings, whole, smashed, or cut up. When cooking, capers should be added towards the end, so that they do not lose their distinctively tart/earthy flavor notes.
In this particular recipe we can add them straight away, because the simmering process is very short.