In language acquisition, one of most important questions is then, how do we acquire language? The research reveals a simple common-sense answer: We acquire language when we understand what we hear or read.
In other words, we acquire language when we receive language input that is comprehensible for us. Language input that is mostly incomprehensible will not turn into meaningful intake that we can acquire.
For example, if you are a new Chinese learner, listening to a native-level Chinese speech, might not be helpful for acquiring Chinese. You may get lots and lots of input—a continuous stream of perfectly pronounced Mandarin, but if the Chinese input that are comprehensible for you were too little, you might not know more Chinese than before. Therefore, an immersion program might not be automatically enough for developing language competence. It has to be both abundant in target language exposure and comprehensible for the students.
On the other hand, if foreign language input is made comprehensible, then it can actually turn into intake that we can process and work with. In this process, students will subconsciously acquire and store it into their long-term memory.