CI, or Comprehensible (understandable) Input, is the teaching method preferred by the Denver Public Schools.
INPUT = language coming IN to your brain: LISTENING AND READING are forms of INPUT
OUTPUT = language coming OUT of you: SPEAKING AND WRITING are OUTPUT, and are the most difficult language skills to achieve.

A good way to explain CI is to think back to your infancy.
When you were an infant, you did not talk. But that did not stop people from talking TO you. That was your first INPUT. As you grew, you continued to receive input from people around you. They talked to you, even though you could not answer them. You just listened and listened. You received a lot of input in your early months and years, but you did not produce any COMPREHENSIBLE (understandable) OUTPUT.

One day, you started to speak. The sounds came out a little funny. The sentences were incomplete. But nobody criticized you, because you were making such a good effort. In fact, the people around you were thrilled to hear you talk!
Imagine if your parent or guardian had been a traditional language teacher...your baby conversations might have gone like this:
Baby: Mama, up!
Mother: Please use a complete sentence, honey.
Baby: Mama, I want up!
Mother: Honey, you need to use a subjunctive clause after "I want". Try again.

Fortunately, most parents and guardians responded to baby's requests by using comprehensible (understandable) input. Like this:
Baby: Mama, up!
Mother: You want mama to pick you up? (more INPUT from mom)
Baby: Mama, I want up!
Mother: OK, let mama put down the groceries and mama will hold you. (lots more INPUT from mom)

So baby got picked up AND got extra input, too! BONUS!!! Eventually, baby learned to speak in complete sentences (and even learned to use subjunctive forms) in time.

My students are Spanish babies. They will receive lots of INPUT from me,the teacher, in the form of listening to me and others use Spanish. Another form of INPUT is reading. We will read a variety of texts, from picture books, novels, memes, ads and poetry. At first, student OUTPUT (speaking and writing) will be very limited and short. As students progress through Spanish levels, they will be able to produce more and more Spanish independently, naturally and comfortably.
In my class, I strive to make students feel comfortable making new sounds and using new words. I strive to give my students input by using mostly Spanish in my classroom. And I strive to make my Spanish comprehensible (understandable) by showing students what the Spanish means, using gestures and translation as supports.

For more information on Comprehensible Input teaching methods, check out the following links: