You can add a description to any checklist item or header, which will display as an expandable drop-down.
To add a description:
Click on an item or header in the checklist to open the inline editor.
Click the menu button that appears on the bottom right.
Click Add description.
On the new line below the >> characters that were automatically added to the text field, enter a description and click Save.
As a shortcut, you can simply click in the text field, press Shift + Enter to create a new line underneath your text, and type >>.