Comprehensible input is language input that can be understood by listeners despite them not understanding all the words and structures in it. It is described as one level above that of the learners if it can only just be understood. According to Krashen's theory of language acquisition, giving learners this kind of input helps them acquire language naturally, rather than learn it consciously.
The teacher selects a reading text for upper-intermediate level learners that is from a lower advanced level course book. Based on what the teacher knows about the learners, the teacher believes that this will give them 'comprehensible input' to help them acquire more language.
In the classroom
Trying to understand language slightly above their level encourages learners to use natural learning strategies such as guessing words from context and inferring meaning. As the example suggests, a teacher needs to know the level of the learners very well in order to select comprehensible input, and in a large class of mixed ability, different learners will need different texts.