。
She must be there by noon.
Literal
She [topic-は] noon [at-に] [contrast-は] there [at-に] must-be.
~なければならない means 'must' or 'have to' — a necessity pattern formed from the conditional negative ~なければ ('if not') + ならない ('won't do'). 正午には uses は to topicalize the time constraint: 'by noon (specifically).' This is a strong obligation statement.