The notice period (Kündigungsfrist) is how long you need to cancel a contract, fire an employee, or quit your job.
All jobs have a notice period, both for the employee (when they quit) and the employer (when they fire someone).1 It depends on your work contract, and how long you work at that company.
Many other contracts have a notice period. For example, if you want to leave your apartment, you must tell your landlord 3 months in advance. There is a Kündigungsfrist of 3 months.
In some cases, you can cancel a contract early if there is a price increase, or if you move to an address where the service is not available.2 This is called a Sonderkündigung.