。
She's holding something in her hand.
Literal
She [topic-は] hand [in-に] something [object-を] is-holding.
何か (something) is the indefinite pronoun — the speaker sees that she's holding something but can't or doesn't specify what. The structure 手に~を持っている marks に for the location and を for the held object, while 何か leaves the object's identity unspecified. This kind of sentence sets up curiosity or narrative anticipation: what is she holding? The bare 何か without a particle here is common in casual speech — the が or を that would technically attach is often dropped.