Comprehensible Input
Listening and reading you can mostly understand — the raw material language acquisition runs on.
Comprehensible input is language you hear or read that you can mostly, but not entirely, understand. The idea, associated with linguist Stephen Krashen, is that acquisition happens when you understand messages, with new pieces riding in on context.
For a beginner the catch is the "mostly": authentic content is far beyond reach at first, which is exactly what the first 1000 words fix. At 85% coverage, simple input becomes comprehensible, and the engine starts. Graded readers, slow podcasts, and subtitled children's shows are the classic first fuels.