不再 (bú zài) means no longer or not anymore. 不 is the negation marker and 再 means again or once more, so 不再 literally means not again. It is placed before a verb to negate continued or repeated action: 不再相信 (no longer believe), 不再回来 (will not come back again). Similar phrases: 再也不 (zài yě bù) and 不再...了 both convey cessation. 不再 is slightly more formal or literary while 再也不 is more colloquial and emphatic.