Content vs. Excerpt
Content refers to the post content. With content mode, WordPress will display everything from the post content unless you specify the more tag. If the more tag is added in the post content, WordPress will cut the post content where you specified the more tag and display a more link on the archive pages. Readers can click on the more link to jump to the single post page.
Excerpt refers to the text in the Excerpt panel. By default, Excerpt panel is hidden. You can turn it on by clicking on the "Screen Options" located top right corner and then checking the "Excerpt" checkbox. The Excerpt panel will appear at the bottom.
If the post Excerpt is empty, WordPress automatically displays the first 55 words from the post content and add [...] at the end. Note that more link and HTML tags such as <img>, headings (h1, h2, h3..), link, etc. are stripped in the excerpt output. If you want to customize the excerpt word counts, read this Custom Excerpt Length tutorial.
Setting Content Display
You can set the content display at Themify > Settings > Default Layouts => Default Index Layout.