You can picture a course list in an American college. Under each subject, the very first line carries the number 101: the class with no requirements, where the teacher starts from zero. Everything this word does comes off that one line — the class itself, and then the idea of anything simple enough to belong in it.
可以想成美國大學的課表。每個科目底下，最上面那一門的編號常常是 101。那是誰都能選、從零開始教的課。整個字的意思都可以從這一格延伸出去：先是那門課本身，再來是「簡單到會被排進那門課」的任何東西。
From the word’s history
Wiktionary records the word as coming from the way United States colleges number their courses, the first or introductory course commonly being numbered 101, as in Biology 101.
Wiktionary 記載這個字來自美國大學的課程編號方式：一個科目裡最初階的那門課常常編成 101，例如 Biology 101。編號被講久了，數字本身就帶上「入門」的意思。
Wiktionary (kaikki dump, source_ref kaikki:3f6f69a190036ae4:1022754121:11120), supplied in the evidence packet