The term “stop the clock” refers to term positions. There is basically a process in place whereby, after one works as a term for a consecutive period of time, after three years, they're turned automatically into full-time indeterminate employees. However, the employer has the right to stop the clock on that if there are reasons why the automatic rollover would not be, I'll say, a good decision.
Stopping the clock basically refers to pausing that time period.