By default, the excerpt (or <description>) element of your feed will populate with the first few hundred words of your post. If you'd like to use your own custom/manual text, you can do this using native WordPress functionality. Here's how:
1) First, let's make sure your Post Editor screen option will display the Excerpt input field. Open your post in Edit mode and expand the Screen Options menu at the very top right of the page.
2) Make sure the "Excerpt" box is checked.
3) Then scroll down below in your post to the "Excerpt" input field and add your own text, save your post, and voilà!