Learn Italian: I ragazzi vanno al supermercato. - The boys go to the supermarket.

In Italian, when the preposition a (to/at) is followed by a definite article like il (the), they combine to form a single word called an articulated preposition. Because supermercato is a masculine singular noun starting with a consonant, it uses the article il. So, a + il becomes al.

Where does the word vanno come from?

Vanno is the "they" form of the verb andare (to go). Andare is a highly irregular verb, which is why vanno looks so different from its base dictionary form.

Does i ragazzi only mean "the boys", or can it include girls too?

It can mean both! In Italian, the masculine plural is used for any mixed group of males and females. If the group going to the supermarket was entirely female, they would be le ragazze. But if there is even one boy in the group, it becomes i ragazzi (the kids or the young people).

Why do we use al (to the) here instead of a form of in?