One of the more important ideas he puts forth is common sense in science, in order for a student to learn the input must be in a 'comprehensible' format. Common sensical though it may sound, there are a number of 'sink-or-swim' educators who stipulate that through sheer exposure and repetition, students will acquire the target skills, in this case, target language. The cartoonish scenario of repeating a word louder and louder to an uncomprehending listener is the opposite of Krashen's 'comprehensible input' premise.
Building on the idea of 'comprehensible input,' for language learners, 'acquisition' is the absorption of the rules of the language via activated osmosis. Krashen believes that language learners must build on their knowledge through 'interlanguage + 1' (i +1). This is also known as scaffolding.
Krashen also penned a piece with Tracy D. Terrell on the Natural Approach (1983), which is more or less a continuation of the comprehensible input theme with the added idea that affective filters, or emotional barriers must be lowered in order for learning to take place.