Keywords are misleading, because they’re not really ‘keys’: they are just the normal, plain language words an author uses to describe his or her text, or words that appear in the title, abstract and elsewhere.
Nonetheless, Keywords are a useful and necessary part of searching. New words for emerging technologies, diseases or abstract concepts are not recognized by databases. You need to use a keyword to find them.
For example: in 2014 a new strain of the flu virus was identified in Asia, the bovine viral diarrhea virus. Articles on BVDV exist in MEDLINE, but the database only recognizes them by looking at keywords.
To search by a keyword type in the word you want and type and add '.mp.'; this tells the database that you want it to search for this particular keyword in many places.
Keywords are very useful, but using them can be a lot of work.
If you want to see every article available on a topic, you will need to think of every synonym that an author could possibly use. A perfectly innocent word like migraine has 35 synonyms in MEDLINE. Some of these might be obscure to the non expert.
Migraines could be referred to using a less specific term, such as 'sick headache', or by older terms such as a 'status migrainosus', or by a very specific term for one aspect of the disease, such as 'Alice in Wonderland syndrome'.
Homonyms, too, are a problem. While depression seems perfectly concrete, a keyword search will also find articles on Cortical Spreading Depression, economic crises, and even compressions in CPR.
You will have to look for all possible synonyms
Keywords may identify a lot of citations that have nothing to do with your topic
Searching by keyword may help you find what your looking for quickly
Keyword searching allows you to find articles on topics not yet recognized by MEDLINE