Similar to PubMed's Medical Subject Headings (MeSH), Embase's Emtree is a set of subject terms that allow you to do a more advanced search than just using keywords (as Google and Google Scholar merely do). The slideshow found below will explain the concept of subject terms like Emtree.
To work with Emtree select Emtree from the top menu. This will open an interface where you can search for specific Emtree terms, or browse using the structured vocabulary.
For example, searching for the Emtree term for cancer will show bring to malignant neoplasm as shown below, or you can navigate to it manually by using the drop-down list and finding your preferred term.
When you find your preferred term you can add it to a search builder (and look for more Emtree terms), and you can set how the term is used. For example, setting it to major focus means the term will be part of the main topic in an article and using explosion will search that Emtree term along with all narrower terms.