Sic enim dilexit Deus mundum ut Filium suum unigenitum daret ut omnis qui credit in eum non pereat sed habeat vitam aeternam. In principio creavit Deus caelum et terram. Et cognoscetis veritatem et veritas liberabit vos.
Normally, an excerpt is shown on the short view of a post (when viewing multiple posts). The excerpt is either manually entered by you or automatically generated from the first few sentences of a post. Instead of showing the excerpt, you can show the first part of your actual post content (including formatting, images, etc.) and control where it breaks.
For example, you are reading the main post content right now (not an excerpt) and the “more” tag has been placed directly after this paragraph. As a result, on the short view, no more content is shown after this paragraph. Instead, a button for reading the rest of the content is shown. The user then clicks that to view the full post.