What a concentration is
A concentration (sometimes called a track or emphasis) is a focused set of required and elective courses within a major — it appears on your degree audit and often on your transcript, but it is not a separate credential from the major itself. Concentrations exist so two students in the same major can graduate with meaningfully different coursework.
How to declare
Concentrations are usually declared with your major adviser once you have enough foundational coursework to choose one meaningfully — often sophomore or junior year. Some majors require choosing a concentration; others make it optional.
Choosing one
Compare a concentration's required courses against your own interests and, if you have one, your intended career path or graduate program. Many concentrations share a core with the base major, so switching between them (or dropping one entirely) is usually less disruptive than switching majors.