Comprehensible input is language content which is understandable to the learner. According to Krashen's Theory of Second Language Acquisition, language learning only takes place when presented with comprehensible input—if you were stranded on a desert island for 20 years with a radio that broadcast only Mandarin you wouldn't learn anything because the language is not comprehensible to you.
Thus, for self-directed language learners, the idea is to find input at your level or slightly above—context will make the unknown bits comprehensible.
Graded readers and Assimil both have this feature.
For beginners, there are a number of ways to make input comprehensible. You could, for example, use translations to help you understand a very simple text in a foreign language, you could do Listening/Reading, or you could interact directly with somebody who speaks the language.