。
She's holding flowers in her hand.
Literal
She [topic-は] hand [in-に] flowers [object-を] is-holding.
手に花を持っている uses the pattern 手に~を持っている for holding something in one's hand — に marks the location, を marks the held object. 花 can mean either a single flower or flowers generally; Japanese often leaves the singular/plural distinction to context. The ~ている marks a current state: she's holding them right now.