My coworker is trying to get the text from the description for a single line of text field using REST. We have so far been unable to find any methods that allows us to access it. We know how to get the description property for URLs/hyperlinks. This is about the description that can be provided for other fields (e.g. single line of text, date and time, etc). We only have REST as an option. It doesn't matter why it is our only option, it just is. So, to reiterate: How do we obtain a field's description using REST?
To get description of all fields, you need to use the below endpoint:
If however, you want to get the data for particular field, you can use it as:
/_api/web/lists/getbytitle('<List-Name>')/fields?$select=Title,InternalName,Description &$filter=Title eq 'YourField'
Reference - Fields REST API