Ethos is a way an author appeals to the reader/viewer. It is one of the 3 appeals...Ethos, Pathos and Logos
Ethos is based on credibility and building trust...so we are looking for facts or things that would make the author worth listening and a credible source.
Example:
Dentists across the world trust and use this toothpaste.
The fact that we know that dentists are endorsing this product adds to the appeal and credibility...it turn we trust it.