The Post settings area of the Administer panel will allow you to choose the settings for displaying posts on your main page (usually the index page).
In this section you can choose how many posts you want to display on a main page. For example, you may want only five posts to show on one page before making a visitor click to another page to show five more.
In addition, you can choose the length of teaser posts. On your main pages, the choice to use a teaser will only display a short excerpt from the article, forcing the reader to click on the post to read on further. This can allow you to show more posts on one page without it being too terribly long, as well as keep track of which posts seem to be of further interest to your readers. These teasers will also be the length of the posts that will show up in XML feeds. To bypass the teaser option, simply select the unlimited option.
Also, on this page it is possible to specify whether a preview of a written post is mandatory or not.