Long tail keywords are keywords that contains more than two words. They usually have less number of searches in Google and hence they are often used to rank webpages higher in search engines.
A long tail keyword is a search term that people type on a search box while searching online. let's say you want to find keto recipes for diabetes, you will type "keto recipes for diabetes" on Google or a search engine of your choice, this is a long tail keyword or long tail search term. Unlike keyword or keywords where you use 1/2 words for searches, you will be using a complete phrase when you make use of long tail search term