Definitions:

1. Refers to the fundamentals or basic elements of a subject, originally derived from the first three letters of the alphabet. NOUN
   • The teacher instructed the students to review their ABC's as part of the language lesson.
2. The rudimentary principles, basics, or elementary components of something, often used in education or metaphorically. NOUN
   • He doesn't even know the ABC's of programming.
   • The children are learning their ABC's.

Synonyms:

• alphabet
• basics
• essentials
• fundamentals
• rudiment