From Halopedia, the Halo wiki
UNSC Ready or Not was a United Nations Space Command frigate. It was patrolling the 18 Scorpii System for Covenant activity when it picked up unusual slipspace readouts. The 6 mile-long anomaly, first suspected to be a Covenant fleet, turned out to be Habitat Exodus, a hollowed-out asteroid equipped with Slipspace drives, escorted by the UNSC frigate Midsummer Night.
The frigate's name appears to be a reference to the phrase "Ready or not, here I come" usually said during a game of hide-and-seek.
List of appearances