As taken from this website, Facebook “uses internal heuristics to make a best guess about the title, description, and preview image for your content.” Within Teachable, this information is specified in a course's SEO section (which can be found in the Course Information page).
Page Title is the heading that appears, while Meta Description is the blurb that appears below the page title. The image shown in Facebook will be the course’s thumbnail, which is set in a course’s Branding section. If you share a link to your school, not an individual course, the image shown will be a collection of your Featured Course’s thumbnails.
If you’ve changed your course information, but the changes are not reflected in Facebook, you may have to use Facebook’s Sharing Debugger.
NOTE: Changes made may take up to 24 hours to propagate on Facebook.
Enter the URL you are sharing into the text box, and click Debug.
If the information shown does not reflect the changes you’ve made, you’ll have to click the Scrape Again button. This causes Facebook to extract the most recent data from the URL — which should then reflect any changes made within Teachable.