Think of subject terms as hashtags that can point you to more sources with the same tags.
For example, this is a screenshot of the hashtag #YosemiteHike found in Twitter.
Finding Subject Terms
When you find a relevant article, make sure to look at the subject terms listed. Think of these as hashtags on Instagram. You can then look for articles with those subject terms to find other potentially relevant items.
Databases also have tools to help you discover subject terms.