Use a html comment with the word "more" to mark the excerpt of your post, like this:
title: post title
This part is the excerpt, get it with the <%- post.excerpt %>
<!-- more -->
This is the rest of the post. You can get it with <%- post.more %>
All the text after the <!-- more --> is only displayed when you view the page of the post.
Edit: You can also use this hexo-front-matter-excerpt plugin to define the excerpt in the front matter with whatever content you want.