Think of subject terms (controlled vocabulary) 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.
When you find a relevant article, make sure to look at the subject terms listed. Many, though not all, databases include them.
Here is an example of some subject terms from the 2015 article "The Effects of Sleep Duration on the Academic Performance of College Students."
Note: Click on the Abstract/Details tab to see the subject terms.