Learn French: Quelques-unes sont fondantes. - A few of them are melting.

Quelques-unes is the feminine plural form of the pronoun. It means 'a few of them' when referring back to a feminine plural noun that was already mentioned (like les bougies, the candles). If the original noun were masculine, you would use quelques-uns.

Can I put a noun right after Quelques-unes?

No, because quelques-unes is a pronoun, meaning it replaces the noun entirely. If you want to keep the noun in the sentence to say 'a few apples', you must use the adjective quelques instead, as in quelques pommes.

Why does fondantes have an -es at the end?

Because fondantes is acting as an adjective here, it must agree with the subject it describes. Since the pronoun quelques-unes is feminine and plural, you add -es to the base word fondant.

Does fondantes just mean literally melting here?

It can mean literally melting (from the verb fondre), but it is also a very common adjective used for food. When describing a dish, fondante means soft, creamy, or melt-in-the-mouth, like a perfectly cooked potato or a warm dessert.