Learn Swedish: Jag brukade träffa min bästis på rasten. - I used to meet my best friend during the break.

In Swedish, bruka functions like an auxiliary or modal verb (like "can" or "must"). This means it is followed directly by the bare infinitive. You just say brukade träffa without an att in between.

Is bästis a slang word for "best friend"?

Yes, it is a very common, colloquial short form of bästa vän. It is similar to "bestie" in English and is widely used in everyday conversation by Swedes of all ages.

Why use på instead of under for "during" the break?

While under is the direct translation of "during", Swedish uses specific prepositions for certain events. With en rast (a break or recess), the idiomatic preposition is always på.

As a quick refresher, why is it min bästis and not mitt bästis?

Bästis is an en-word (en bästis), so you must use the matching en-word possessive pronoun min. If the noun had been an ett-word, you would have used mitt.