Bilingualism is when people speak in two differents languages, and that could be for the parents because if the baby is born in Chile and her mother is Chilean too but her father is french the baby could speak in Spanish and in French too.
Another reazon is because the person had the oportunity of traveling to other country for many years and he learned to speak another language apart from his mother tongue, or just because the person wants to learn another language and in that way the person is a bilingual person.
In my opinion I would have like learn another language when a was a child and all this would be easiest for me now.