A keyword search looks for items that contain that word or phrase in the title, abstract, description, text, etc. Different writers use different terminology, so be sure to think of synonyms or similar words. If you search for purchasing, you won't get papers that use the term buying instead. To search for both terms, you can use the OR boolean: purchasing OR buying.
Booleans are words that are used to connect search terms-
OR will broaden your search by searching for items with either term: purchasing OR buying will retrieve papers that have at least one of the words.
AND will narrow your results by only searching for items that contain both terms: purchasing AND overhead will only retrieve papers that include both words in the text.
Subject headings are similar to tags in that they are assigned to works that are related to the topic. Unlike a keyword search, subject heading searches will find books that don't necessarily have the term in the title or abstract but are about that subject. You can think of a subject heading results list as a list of recommended books on a topic.