To be supportive is to give help or assistance, to support. Good bras and good friends are supportive. Whether it's an object or a person, anything or anyone supportive helps things hold up.
Supportive is a snuggly word. Anything that supports you, or embraces you and holds you up is supportive. It can be physically supportive, like a girdle that holds your belly in, or emotionally supportive like a loving family or solid network of friends. A parent paying for a child's college bills is supportive financially. Things can be supportive, too — posts holding up a building are supportive. Being supportive is a good thing.