introductory
/ˌɪntrəˈdʌktəri/ adjective
an introductory chapter/essay/message, etc › a part that comes at the beginning of a piece of writing or a speech and explains what will come later
an introductory book/course/lesson, etc › something that provides basic information about a subject: an introductory course in economics
an introductory discount/fare/offer, etc › something that you get when you start buying something or using a service