Keywords and keyword phrases are just words or a word phrase. Often the term “keyword” includes phrases. Keyword phrases are also called keyphrases, long tail keywords or long tail keyword phrases. I prefer to use the term Keyword to mean both single and multiple words.
Bottom line, keywords are words and phrases that the search engine analyzes for their indexes and what people type into the search engine to find the information they want. Examples of keywords are: gardening, organic gardening, herb garden, gardening tips, gardening supplies.
In this usage, what is important is how often the word or phrase was searched in search engines, such as Google. In other words, how often did people type into Google that word or word phrase to find the information they are looking for.
So if I am writing about antique furniture, if I use ‘old’ as a keyword, it wouldn’t work so well as I would get people searching for old hats, old dogs, etc. Yet I could use a keyword phrase of ‘old furniture’ if it ranks well in the search engine listings.
Keywords, in this usage, has little to do with the older copywriting concept of a key word or phrase in the article. Think of it as that humans do not read keywords initially. It’s all about how people will search the internet and find the article, so the computer reads it first.