An interview cannot truly tell if the hire is a perfect fit for the role or not. Hence, when an employee is kept on probation period at the time when the offer letter is rolled out with the contract of employment. Probation is an agreed upon formal agreement. In this agreement, a particular time frame is decided by the employer during which the employee has to demonstrate his worth for the job.
Depending on the company norms, the probation period can be anything between 30 to 180 days. This is the time when the employee can be ousted from the organisation if his work or behavior is not found to be good enough.
Please note that employers cannot impose probation again on the employees after they have successfully completed it. However, if the original contract allows them to, they may use it again.