In English, saying The trains are big or I like the wine usually means you're talking about a particular wine or specific trains. In French, you use le, la, or les when talking about specific things, but you also use these words when stating general facts!
You also use them when talking about your likes or dislikes.
And also when talking about countries!
We've already seen that au is a combination of à and le. Now, if the noun is plural, use aux, instead of à les.
à + le = au
à + les = aux
Here are some examples that use these contractions.