In the editor's «Blocks» tab, press «Add Block» and choose «Divider».
Choose the line style (solid, dashed or dotted) and its thickness. The colour is optional: left empty, it follows the theme.
The preview on the right changes as you type. When you are done press «Save»: until you save, visitors see the previous version.
Still stuck?
Terms and definitions live in the glossary; for anything else there’s support.