It is important that when you start a new job you fully understand what you are being asked to sign. Your contract should include information about pay, holidays, sick pay, bonuses, commissions, pensions, share options, disciplinary and grievance procedures, what happens if you leave and any restrictive covenants, or non-compete clauses.
When you start a new job you do not accrue the right not to be unfairly dismissed until you have worked for two years. As such, it is even more important in those early years that you are protected, so far as possible, by your contract.
Restrictive covenants often exist to prevent you from competing against your employer should you leave their business. Breaking a valid restrictive covenant can have serious consequences, so always ensure you know what you are signing and what effect they will have after the termination of your employment.
If you or your employer wants to change something in your contract, you both have to agree. There are some exceptions if, for example, your contract includes a flexibility clause allowing changes to certain conditions, or a mobility clause allowing changes to your usual place of work.