Introduction (also known as a prolegomenon) is a beginning section which states the purpose and goals of the following content. The introduction typically describes the scope of the content and gives the brief explanation or summary of it. — Wikipedia
Prolegomenon (usually plural prolegomena) is an Ancient Greek word used to mean “prologue” or “introduction”, to introduce a larger work, e.g., a book. — Wikipedia
Threshold knowledge is a term used to describe core concepts—or threshold concepts—which, once understood, transform perception of a given subject, phenomenon, or experience. The term was Introduced by Jan Meyer and Ray Land. They also discuss the related idea of troublesome knowledge, ideas that appear alien or counter-intuitive. — Wikipedia
Propaedeutics or propedeutics (from Ancient Greek προπαίδευσις, propaídeusis ‘preparatory education’) is a historical term for an introductory course into an art or science. The etymology of propedeutics comprises the Latin prefix pro, meaning earlier, rudimentary, or in front of, and the Greek paideutikós, which means “pertaining to teaching”. As implied by the etymology, propaedeutics may be defined more particularly as the knowledge necessary before, or for the learning of, a discipline, but not which is sufficient for proficiency. — Wikipedia
Teaching Introductory Courses: What Is the Public Face of Your Field? (Maryellen Weimer, Faculty Focus)
What are Gateway Courses and Why Do They Matter to Equity in Higher Ed? (Jessie Kwak, Every Learner Every Where)
Here are links to pages about closely related subjects.