There is an option that allows a text pop-up to be displayed if a keyword defined within the note is hovered over with the cursor.
Every time that keyword appears it will have a dotted underline and will trigger the pop-up if hovered over.
The markdown required takes the format:
*[trigger word]: pop-up text
The feature can be found in Joplin settings under Markdown > Enable abbreviation syntax (wysiwyg: no).
By its name you can see that this feature is actually intended to allow abbreviations to be defined such as
*[HTML]: Hypertext Markup Language, so it may be a bit limited.