Comprehensible input
Language input you mostly understand, with a little stretch. The cleanest single concept in language acquisition theory.
What it means
Comprehensible input is language you can understand. Spoken or written, in the target language, at a level where you mostly follow what's being said but encounter some unfamiliar pieces. The unfamiliar pieces are where learning happens — but only if the surrounding context lets you guess at them.
Stephen Krashen put forward this idea in Principles and Practice in Second Language Acquisition (1982) and built much of his theory around it. The claim: input that is comprehensible is necessary and sufficient for language acquisition. Output, drills, error correction — those help, but they don't substitute for input.
Why it works
When you hear or read a sentence and almost-but-not-quite understand a word, your brain is doing implicit work to fill the gap. Over hundreds of repetitions across different contexts, the gaps get filled in. Not by deliberate memorization — by accumulated exposure. This is how children acquire their first language and how adults acquire their second when given enough time and material.
How it applies to reading
Reading is the densest delivery system for comprehensible input adults have. A 250-page novel gives you something like 80,000 words of language, mostly in patterns you've seen before, with a few thousand new ones. The story keeps you going. The repetition is built in: the same words appear in scene after scene. Provided the book is roughly the right level, every chapter is a small experiment in comprehension.
The trick is matching the level. A book that's far too hard isn't comprehensible — it's noise. A book that's far too easy doesn't push you. The right book sits at i+1: your current level, plus a little.