Second language is a word given to a language that a person learns in addition to their first language. A second language may be learned in a formal or informal way, e.g. at school or in a family. It is possible that a person speaks two or more second languages.
Adults do not learn languages the same way they did when they were young. So if they learn a second language, it is difficult, and they have little chance of speaking it as well as their native language because learning a second language will require a lot of effort.
Some children have native multilingualism. For example, they live in a foreign country or their father speaks a different language from their mother.