The simplest explanation is that Cyclescheme acts as an employee benefit, in the form of a salary sacrifice.
This means that you agree to give up part of your salary in exchange for a benefit – in this instance the benefit is a bike and/or accessories. The salary sacrifice is taken from your gross salary (before tax) which means that you will pay less Income Tax and National insurance.
It is worth noting that there is no credit check if you wish to participate, and if you are under 18 you can join the scheme if your guardian signs a guarantor agreement. Being on probation should not affect your eligibility — if you are unsure, please check with your employer.
Cyclescheme have produced a particularly helpful video to help explain this.