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 standards, you won't get papers that use the term guidelines instead. To search for both terms, you can use the OR boolean: standards OR guidelines.
Booleans are words that are used to connect search terms-
AND will narrow your results by only searching for items that contain both terms: standards AND personnel management will only retrieve papers that include both words in the text.
NOT will narrow your search by excluding results that contain that term: leadership NOT management will retrieve papers that discuss leadership but not management.
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.