A subject search uses the full item record to figure out how that item is described by the library. You can then use that linked-description to find items on the same subject.
First search by keyterm(s) in the Basic Search box on the library homepage. For this example, we will look at the search phrase "climate change" AND "health":
Next choose the most relevant result and open the full record by clicking the title:
Once you have opened the full record, scroll down to the "Details" section:
There is a lot of information in this section, but the part you need is next to the heading "Subject":
Of the eight subjects assigned to this book, "Climatic changes-- Health aspects-- United States" is probably the closest to the subject of climate change and health. To view other items assigned the same subject, just click "Climatic changes-- Health aspects-- United States" and a new set of search results will appear that are all assigned the same subject phrase.
Also look at the subjects assigned to the new results. You may find a related subject that fits even more closely with your topic. For instance, the book highlighted in the following image is described with the subject "Climatic changes -- Health aspects -- United States", but also the subjects: