Learn Italian: L'anno scorso si sono evitate le giacenze. - Last year, unsold stocks were avoided.

This is the si passivante (passive si) construction. In this structure, the thing being acted upon (le giacenze) acts as the grammatical subject. Because le giacenze (unsold stocks) is feminine plural, the auxiliary verb must be plural (sono) and the past participle must match its gender and number (evitate).

The verb evitare normally takes avere. Why does this sentence use essere (sono)?

Whenever you use any si construction in a compound tense—whether it is the si passivante used here, an impersonal si, or a reflexive verb—the auxiliary verb is always essere. This rule overrules the verb's normal preference for avere in the active voice.

Why does the subject le giacenze come after the verb?

While you could absolutely say L'anno scorso le giacenze si sono evitate, Italian very often places the subject after the verb in si passivante constructions. This word order sounds highly natural and puts the emphasis heavily on the action itself (the fact that they were avoided) rather than on the stocks.

What exactly does the word giacenze mean here?