Ethos is how the character, sincerity and credibility of the speaker affects the audience.
The first of Aristotle's three parts of rhetoric, ethos is an essential idea for public figures and public relations professionals.
If your credibility is damaged -- by errors, omissions, miscommunications or acts of God -- nothing else matters. The press and public won't listen.
Here are some of the building blocks of ethos:
Authority
Is the speaker and expert on this issue?
This doesn't have to be an academic degree or a professional position. Maybe the speaker is a surfer who survived a shark attack. You'd listen to them if they gave a speech on sharks, wouldn't you?
Fairness
Does the speaker have a stake in the outcome, or are they impartial?
Audiences are more willing to believe and be persuaded by someone who doesn't have something at stake. This is why you listen to your friends and neighbors about issues when they don't have a dog in the fight, and why you tend not to trust used car salesmen.
Reputation
Does the speaker have a track record of being honest and correct?
If you're the boy who cried wolf, it doesn't matter if you have photographic proof that the wolf is eating chickens at grandma's farm and bringing six of his friends tomorrow. Credibility is essential. It's hard to earn and easy to lose.