If you have a lot of posts in your blog, you may want to have only excerpts on your homepage that link to the full blog post. Otherwise, your readers will have to scroll thru an entire blog post, which may be many screen blogs long, before viewing the next blog entries’ content (his also leads to the topic of making a good blog entry title that describes the content of the blog entry, but that’s another topic!).
To make short excerpts on your homepage, click on the “Insert More Tag” toolbar item when creating or editing an entry. Doing so cuts off the content where your cursor is at. Ideally you’ll want to have a nice intro paragraph that would be featured on the homepage, then more content you click on the link.