101

From the practice in US colleges of numbering courses, the initial (introductory) course commonly being numbered "101".

Full definition of 101

(chiefly US, often postpositive) Basic, beginner, starting from scratch.
Geology 101 tells us that you can't build a reservoir on sandstone.

2004, Daniel Dor, From Englishization to Imposed Multilingualism: Globalization, the Internet, and the Political Economy of the Linguistic Code, Companies sell directories, databases, reports, translation services, automatic translation software, and guidebooks for doing business away from home, which, in some cases, look much like simplified textbooks for Anthropology 101.
2023-03-20, Rebecca Gillam, Bridie Wilkins, Alexandra Daddario and her PT on how she built her Baywatch bod + her 10 regular workout rules, However much you work out, but especially if itâ€™s a lot, learning correct form and focusing on keeping your body safe is 101 â€” not just while exercising, but also how you move around day to day.

There is a difference in pronunciation depending on the person. Many people still use "one hundred and one", which also applies to almost all numbers after 101. However (outside Britain) it is becoming common to hear the "and" omitted, simply "one hundred one".

Symbolizing more than 100, an already large number.

A large number, particularly the size of an exhaustively long list of examples, uses, applications, etc.

1996 , Samela Harris , On a Shoestring: Recipes from the House of the Raising Sons Chapter Introduction