Learn Italian: Mi piacciono i biscotti.

In Italian, the verb piacere (to like) works a bit backwards compared to English. You are literally saying 'Cookies are pleasing to me'. Because biscotti (cookies) is a plural word, you must use the plural verb form piacciono (they are pleasing) instead of the singular form piace (it is pleasing).

What exactly does Mi mean here?

Mi means 'to me'. In Italian, you don't literally say 'I like something'. Instead, you say 'Something is pleasing to me'. So instead of using a subject word like io (I), you use mi to show who is experiencing the feeling.

Why do we need the article i before biscotti? We just say 'I like cookies' in English.

In Italian, when you talk about a general category of things you like or dislike, you must always include the definite article (the word for 'the'). Since biscotti is a masculine plural noun, it needs the masculine plural article i.

How does the word biscotti change if I only want one cookie?

Most Italian nouns that are masculine end in -o in the singular and change to -i in the plural. So a single cookie is un biscotto, and multiple cookies are i biscotti.