If in Appearance → Customize → Blog settings → Posts list → Type of post excerpts is set to Automatic, then if while writing posts you won’t use more tag, then theme will use excerpt feature.
Excerpts are made automatically, but you can also specify what will be displayed. Just find Excerpt box, and type text you wish – read more.
If you won’t give excerpt either, then post will be cut after first 40 words. This number can be changed in Appearance → Customize → Blog settings → Posts list → Number of words to cut post.