Learn Spanish (Spain): ¿Dónde están los platos?

No, in this sentence los is just the plural definite article meaning the, because it comes right before the noun platos (plates). The direct object pronoun los (meaning them) looks exactly the same, but it usually goes right before a verb.

Why do we use están here instead of son?

We use están (from the verb estar) because the sentence is asking about the physical location of the plates. In Spanish, you always use estar to talk about where things or people are located, while ser is used for general descriptions or characteristics.

Why is there an accent mark on Dónde?

All question words in Spanish require a written accent mark when used in a question. Writing Dónde with an accent distinguishes it from the word donde used in normal statements.