Comprehensible input is a term Steven Krashen uses a lot.
We acquire language in one way and only one way: when we understand messages. We call this comprehensible input. We acquire language when we understand what people tell us. Not how they say it, but what they say. Or when we understand what we read.
Understanding spoken and written language input is seen as the only mechanism that results in the increase of underlying linguistic competence.
In practice comprehensible input is some content in a foreign language. It can be anything, text, audio, video. The content contains something new, something you don't understand yet. However, it is prepared in a way that makes unknown bits comprehensible. Or you can make it comprehensible yourself by unlocking the unknown bits.
Anything can be used as a tool to make input comprehensible. For example, images. Comic books are more comprehensible than novels because they contain pictures. Pictures give you clues, hints, help you understand what's going on in the story. Cartoons, movies are even more comprehensible. By paying attention to how people talk, how they're moving, what gestures they use you can understand what they're saying better.