The history and the origin of words and phrases.
Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time. For example, this area of research answers questions like "How did word X get its current meaning?", or "Is this word a borrowing from another language?".
Note that along with the actual etymology a word or a phrase may have a so-called "folk etymology", which occurs when native speakers don't see the origin of the word and instead re-analyze it. A famous example of such folk etymology is saying that "hamburger" comes from word "ham" and "burger", when it originally stems from "Hamburg".
If you are interested in folk etymology instead of the real one, please state so in your question.