Students need decodable language input, that is, reading and listening, at their level, on topics that interest them. Explanations of grammar rules and time spent correcting students don’t count!
Examples of input include stories, songs, games, and books (storybooks, not textbooks!) that are at the right level for learners of the language.
Q: This can’t be possible for true beginners, can it?
A: Actually, it can!
Once at a CLTA conference, the presenter had us all playing Red Light, Green Light in Japanese to demonstrate games in the target language. While hardly any teachers in the room spoke any Japanese beforehand, we all came out knowing "aka” for red and “midori” for green. This is an example of how input can work even for true beginners, and how the context matters: You learn to understand it as you do the activity, not because it's something you have to do.
Q: Where do teachers get input from?
A: They usually create it themselves, but there are also comprehensible novels written at students’ level by other teachers, children’s books, and existing songs, games, and infographics in the language, if they know where to look.
Q: Is input only for comprehension? What about learning how to speak?
A: It’s both! Input is the basis for acquiring enough language to be able to communicate one’s own thoughts as well as understand others. Of course, opportunities to try out communicating in the language are also important, as addressed in the rest of the sections of the Science of Languages.
For further reading about input: