Learn Swedish: Är sakerna som du samlar på värdefulla? - Are the things that you collect valuable?

In Swedish, when a relative clause is introduced by som (that/which), any preposition connected to the verb stays at the end of the clause. You cannot say "på som" (on which). Instead, the preposition is "stranded" at the end, giving us som du samlar på.

Why do we need the word på here at all? Doesn't samlar mean "collect"?

There is a slight difference in meaning. Samla on its own usually means "to gather" (like gathering people in a room or papers into a pile). When you talk about collecting things as a hobby or habit, you use the phrasal verb samla på.

Why does värdefulla end in an -a?

Because it describes sakerna (the things), which is plural. Even though the adjective is separated from its noun by the som clause, it still has to agree with the noun it describes. Therefore, värdefull takes the plural ending -a.

Why is värdefulla at the very end of the sentence?

Because the main question of the sentence is simply Är sakerna värdefulla? (Are the things valuable?). The entire relative clause som du samlar på (that you collect) is just extra information inserted into the middle of the sentence to specify exactly which things are being discussed.