A bilingual person is someone who speaks two languages. It's possible for a person to know and use three, four, or even more languages fluently. People may become bilingual either by acquiring two languages at the same time in childhood or by learning a second language sometime after acquiring their first language.
Many bilingual people grow up speaking two languages.
In short, a young child who is regularly exposed to two languages from an early age will most likely become a fluent native speaker of both languages. It is also possible to learn a second language sometime after early childhood, but the older you get, the harder it is to learn to speak a new language as well as a native speaker. Under this view, the structure of the brain changes at puberty, and after that it becomes harder to learn a new language. This means that it is much easier to learn a second language during childhood than as an adult. In some countries, nearly everybody is bilingual or multilingual.