The probationary period is a period at the beginning of an employment relationship during which both the employee and the employer can end the employment relationship more easily than later. The maximum length, the way it is arranged and other rules are set by legislation — and it differs over time and between countries, so always verify the specific conditions with an accountant or lawyer.
From an HR point of view, the main thing is that the end of the probationary period does not pass unnoticed for any employee.