A notice period in employment is the duration which needs to be given to the employee by the employer or to the employer by the employee before the employment can be terminated. An employee can be terminated sooner if they breach their agreement.
A notice period in employment will vary based on the industry, the company and the role. The length of the notice period should be reasonable which means long enough for the employer to find a replacement or the employee to find a role but short enough that it does not lock either side into an unwanted relationship.
An employee might be requested to work during their notice period. This might mean helping find and train their replacement or finishing projects they're working on. If an employer doesn't want the employee to work their notice period they can place them on garden leave. Alternatively, the employee and employer might opt for a payment in lieu of notice (PILON) which means the employee will be paid at least the equivalent of their salary for the period of their notice and the employment will be terminated immediately.
An employees restrictions will usually apply to their notice period as well a period after their employment. Moreover, the employer's rights will continue to apply during the notice period which means for example that the employer would normally own any intellectual property developed during the notice period by the employee.
If a candidate for a role is already an employee of a different employer, then they will need to terminate their employment by giving notice to their existing employer before they can start the new role. The candidate might therefore be asked by to provide their notice period in the job application.