Those are breasts. They're filled with hopes and dreams.

These are human form of mammary glands. As a female pony you had posterior mammary glands (also known as crotchboobs). Female humans have anterior mammary glands (simply called breasts). As you probably know their primary function is production of milk. Unlike stallions, male humans also have mammary glands, which usually don't produce milk. Female breasts are larger and usually considered "private" part of human body, so you shouldn't show them in public. Though I won't be offended if you do so or even let me touch them…