"Preemtion is the process of forcibly moving a thread from running state to ready state when a higher priority thread become ready to run" Simple Real-time Operating System: A Kernel Inside View for a Beginner, Chowdary Venkatesware Penumuchu, 2010.

When a thread is preempted, its context is saved in its TCB and then this thread becomes frozen. Then, the higher priority thread context is loaded and its execution is resumed.