。
She's taking piano lessons.
Literal
She [topic-は] piano [object-を] is-learning.
習う specifically means 'to learn from a teacher' — to take lessons under instruction. It's distinct from 勉強する ('study, work at'), which is broader and can include solo study, and from 学ぶ ('study, learn'), which leans toward formal academic learning. The progressive 習っています here describes an ongoing arrangement, not a single act — the standard 'is taking lessons' frame for any regularly scheduled instruction.