。
You have to keep your promises.
Literal
Promise [topic-は] keep-[must-なければならない].
~なければならない is the slightly more formal variant of ~なければいけない, both meaning 'must.' ならない (from なる) is more abstract than いけない (from いく, 'go'). Functionally interchangeable for most purposes; ならない is preferred in written and institutional contexts.