Welcome to the new block editor. You are looking at a paragraph block. At the end of it there is a more block. If you’re reading this text from an archive you can see a link to read the full post following the paragraph. This one for example shows “Keep reading” but the default value is “Read more”. The text of the read more link can be customized from within the editor.
Keep reading →Gutenberg Blocks
Jetpack blocks
Jetpack adds a number of blocks to the new editor. For example this is the markdown block. You can show text in italics by wrapping it around asterisks or in bold by wrapping it around double asterisks. Alternatively you can strike text through or display inline code. It doesn’t stop there though. The markdown syntax is quite comprehensive. Use this cheatsheet to get started.