It’s a fixed form: “Réalité indéfinie mais qualifiée par une précision subséquente. Quelque chose de + adj. inv. Quelque chose de beau, de bon, de grand, de vrai, de vague, de bleu, de neuf, de nouveau, de fâcheux, de merveilleux.”.

The definition means this : given an instance of this form, it specifies something that is not defined but that has one characteristic, that characteristic being embodied by the adjective ; therefore from the context you might more often than not understand what that something is.

If the person’s at a dressmaker and you hear someone saying “Je voudrais quelque chose de simple.” you know the person wants a dress that is simple ; there is nothing to that, really, it’s the same in English ; the only difference is that in English there is no preposition used in the corresponding form (example: I’d like something simple on this table, maybe a mere flowerpot with pansies in it.)

It is a common construction built this way :

1. a pronoun

2. the preposition de

3. an adjective, past participle, comparative and a few other qualifiers

e.g. :

Je n’ai rien de cassé.
C’est quelqu’un d‘intelligent.
Quoi de mieux qu’un bon repas.
Rien de neuf sous le soleil.
C’est tout ce qu’il y a de plus français.