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 outdoor recreation, you won't get papers that use the term adventure instead. To search for both terms, you can use the OR boolean: recreation OR adventure.
Booleans are words that are used to connect search terms-
OR will broaden your search by searching for items with either term: recreation OR adventure 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: leisure AND sustainability will only retrieve papers that include both words in the text.
NOT will narrow your search by excluding results that contain that term: endurance race NOT racism would be a way to retrieve papers that discuss the concept of 'race' as a sports event, not the cultural construct used to classify human beings.
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.