what does postman mean when he says "...what ideas are convenient to express become the important content of culture

Postman is referring to methods of communication.......... you really shortened this quote, but its meaning is that the way we exchange messaged, be it through speech or technology influences the beliefs of the audience and affects the believability of the speaker.

He uses the example of smoke signals, once a preferred method of communication for Native American tribes. It didn't matter who was 'speaking,' the only thing that mattered was the message. Television changed this, and again, he uses a brilliant example........ that being our 27th President. Postman cites that if Taft were running for President today his appearance would make his running implausible. The man had multiple chins, weighed 300 pounds, and his image would completely turn our media based audience off. Television relies on appearance, visual imagery, and we tend to embrace beauty before message. This is in complete contrast to a message sent in smoke signals...... what someone looks like doesn't matter when you're listening to the radio, sending a telegram, or even sending smoke signals, but it does matter when you see a person's physical appearance on television or some other form of media. He makes a great point, as this can be seen in the number of low quality, popular programming available today. Many of the highest ranking shows are nothing more than garbage, but the 'view' and the 'characters' are beautiful to look at.