In the 1970s and 1980s, the linguist Steven Krashen developed several hypotheses of second-language acquisition, including what he called the Input Hypothesis. He hypothesized that we acquire languages by being exposed to “comprehensible input,” spoken and written language that we can understand. The following video explains how it works:
Comprehensible input combined with spaced repetition is considered by many to be the fastest way to learn a new language.