A select control lets someone choose an option from a list. The list is usually tucked away until the person opens it, which saves space on the page. It is like choosing one item from a menu after the options are shown.
For example, a form might ask which country you live in. The person opens the list and picks one country. A select control is useful when there are enough choices that showing every option at once would take too much room.
Keep option names short and easy to tell apart. If there are only two or three choices, showing them all directly may be simpler than hiding them in a menu.