A probationary period refers to a time at the beginning of employment during which the employer is assessing if their recruit is a good fit for their new role. Specifically, it releases some of the usual contractual obligations of employment and notably changes the notice period for termination. In other words, it allows either party to exit the employment contract more easily and without a lot of notice.
Probationary periods are commonly between three to six months, but some companies stretch this out to an entire year. It is widely accepted that part-timers or contract workers may be given shorter probationary periods.