Authentic materials are written or spoken texts used with learners without changing the level of language.

Example
A story from a news webpage, streamed radio, a story from a CDROM, a podcast, or a live native speaker talking to learners without adjusting the level of their language.

In the classroom
Authentic materials can be exploited at different levels by changing the level of the task to suit learner levels. Thus, the same piece of authentic material can be used with an elementary level class or an advanced one.

Further links:

https://www.teachingenglish.org.uk/blogs/anthony-ash/authentic-listening-genuine-tasks