I'm assuming you are referring to the Declaration of Independence.
Ethos is an appeal to ethics. The principle of it is to prove to the reader that the writer is credible and reasonable. In the Declaration of Independence Jefferson does this very skillfully in the first line when he says that the declaration is written out of respect for those that must judge the rightness or wrongness of the colonies decision to break away from Great Britain. This establishes his credibility.
Since you have not given any reference to the declaration you're talking about, i'll just go and try to help you how ethos could appeal to a speech or essay. Ethos refers to the credibility or personality/character of the speaker. For example, someone who has been through the World War II can be appeal to ethos when it comes to his experiences in it and to persuade you to do diplomatic negotiation instead.
