A restrictive covenant is a written term found in the employment contract which seeks to prevent an employee from carrying out a particular activity, usually in competition with their employer, after the employment has ended.
A contract change to introduce a restriction of this kind cannot be imposed. It needs your consent.
Normally, that consent will be 'express' – i.e. clear and in writing. However, be aware that you can sometimes be taken to have accepted this kind of change through your behaviour, even if you did not sign and return the written contract.
In practice, it is usually sensible to be up front with your employer when asked to agree a contract term that you do not agree with, such as a new restrictive covenant, and to try to negotiate sensible limits, or else something in return.
You could, for example, suggest a 'gardening leave' clause instead. Under a typical gardening leave clause, you will serve out your notice at home.
This kind of term enables the employer to stop you competing while on gardening leave, but is better for you than a restrictive covenant because even though you cannot join a competitor while on gardening leave, you are still getting paid your wages.