Example
How is sleep affected in people with brain injuries?
Step 1: Break up this research question into two or three main concepts. Ignore extraneous words.
Step 2: Conduct an initial search with these words, and quickly scan the results.
Step 3: Using information from your initial search, add synonyms, acronyms, and variant spellings.
(sleep OR insomnia OR hypersomnolence) AND (brain injury OR tbi OR concussion)
You are probably most used to keyword searching, in which you brainstorm search words and various synonyms.
Subject searching can be used in addition to or as an alternative to keyword searching. If you use the correct subject heading, you will capture synonyms automatically (e.g. "brain injuries" will find "brain injury" and "concussion"). How do you know what the correct subject heading is? You can look them up in many databases.
There are people whose job it is to read articles and assign subject headings to them (i.e. decide what they are "about"). It's kind of like assigning a hashtag to something in Twitter and then being able to search by that hashtag (...kind of).