A podcast is an electronic program that is delivered to subscribers via an RSS feed. It can be audio files (typically MP3's), video, or documents or any combination of the three. For the purposes of your US History podcast it is a short MP3 presentation that you will share with your teacher that functions in much the same way that an essay would. You should have a thesis which presents your argument and then evidence that supports that argument. However, the podcast is not you simply reading from a paper. Yes, you have a script but think of podcasting like storytelling. You weave in quotes, documents, interviews, etc with your narration that holds all of these things together. If you have ever listened to NPR a lot of their pieces are like podcasts. Listen to some of the examples provided for a sense of what makes a podcast. There is usually a great deal of production value: background music, integrated sound effects, fading in and out of voices, etc. However, all of these bells and whistles still won't make up for a lack of content. Make sure you have a point to your story and that everything you include in your podcast reflects that point!
Here is an example of a podcast from NPR's "This American Life".