。
She couldn't help but laugh.
Literal
She [topic-は] endure-can't-completely laughed [ended-up-てしまった].
こらえる ('to endure, hold back, suppress') paired with the auxiliary ~きれず ('without being able to completely [verb]') gives こらえきれず ('without being able to fully suppress'). ~きれない / ~きれず attaches to verb stems and means 'cannot do X completely / cannot finish X-ing.' Combined with 笑ってしまった (~てしまう = 'end up doing,' often with regret or unintendedness), the whole sentence captures a moment of involuntary laughter — she tried to hold it in, failed, and burst out laughing despite herself. A vivid frame for the failed-suppression-of-emotion experience.