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. If you search for concussion, you won't get papers that only use the technical term traumatic brain injury. To search for both terms, you can use the OR boolean: concussion OR traumatic brain injury.
Boolean search operators describe the relationship between your search terms-
Narrows your search to just those articles that have both words in the text.
Broadens your search to those articles that have at least one of the words.
Narrows your search by excluding the word following NOT. This search will retrieve papers that discuss tapering in the context of competition, not along racial lines. NOT is easily misused. Only use it when you're having problems with your results. Be sure you aren't inadvertently excluding results that would be useful. It's most often better to narrow your search by adding terms rather than by excluding terms.
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.