Learn Farsi: این محافظ یکی از لوازم جانبی مهم است. - This protector is one of the important accessories.

لوازم (lavâzem) is the Arabic broken plural of لازمه (lâzeme), meaning "necessity" or "equipment". جانبی (jânebi) comes from جانب (jâneb - side/flank) plus the relative suffix ی, meaning "peripheral". Together, they literally mean "peripheral equipment", which is the standard Farsi phrase for "accessories" (for electronics, cars, etc.).

In the phrase لوازم جانبی مهم, why does مهم come at the very end, and how are these words linked?

In Farsi, when a noun takes multiple adjectives, classifying adjectives (like جانبی, which tells us the type of equipment) stick closest to the noun. Descriptive adjectives (like مهم, which describes its quality) come last. They are linked by the ezafe: lavâzem-e jânebi-ye mohemm. Notice the -ye instead of just -e because جانبی ends in a vowel.

Does یکی از (one of) strictly require a plural noun to follow it?

Yes, just like in English, یکی از must be followed by a plural noun or pronoun. In this sentence, the requirement is met by لوازم, which is already plural. If you were using a standard Farsi noun, you would see the plural suffix, as in یکی از ماشین‌ها (one of the cars).

What is the root of the word محافظ (protector)?