Questions & Answers about Das Einzige, was ich heute brauche, ist Ruhe.
It means the only thing.
German often uses an adjective as a noun. Here, einzig means only or sole, and das Einzige literally means the only one / the only thing.
In this sentence, das Einzige is the subject of the whole sentence:
Das Einzige ... ist Ruhe.
= The only thing ... is peace/rest/quiet.
Because it refers to a thing or idea rather than a person, it is treated as neuter, which is why it uses das.
Because it is being used as a noun.
Normally, einzig is an adjective:
- der einzige Grund = the only reason
But in your sentence, the noun is not stated explicitly. German allows the adjective to stand on its own and act like a noun:
- das Einzige = the only thing
When adjectives are turned into nouns this way, they are capitalized in German.