The is the most popular word in the English language. Since it is so famous, the word "the" must be in every sentence on this page, but that's not very difficult. Headers, template titles, and categories, however, do not have to follow the rule as they are not true sentences.
People seriously shouldn't even need to know what "the" means. The meaning of it can be defined peculiarly, but why would you want to be confused? It should be one of your first words, so you probably wouldn't understand the other words in its definition yet.
The word "the" functions as an determiner, but more specifically as an article (or article adjective). If you can't define "the", you definitely can't figure out what that means, and you probably can't read either.