An association links an article to its translation. It is what lets the language switcher take you to the RIGHT page, rather than to the home page.
Where it is set
In the article, under the “Associations” tab. For each other language on the site, you pick the matching article. A menu item works the same way, and so does a category.
This article has one: switch language with the selector, and you will land on its translation, not on the home page.
It is not a translation
Joomla translates nothing. An association only says “these two articles match”. The text of each language stays hand-written, and may well say something else: that is often the better choice, since one subject is rarely told the same way from one country to the next.