 Basically, a pedal sensor is a device which makes sure that the pedals are turning - if they are, it enables the throttle, and if they aren’t it cuts power to the motor.
So, in effect, the electric motor will only work if you are turning the pedals. A bike of this sort is called a Pedelec - a bike without a pedal sensor is called an E-bike. Under the new EU rules, a pedal sensor is required for the bike to avoid Single Vehicle / Whole Vehicle Type Approval - this is the complex (and expensive) approval scheme used for motorbikes and mopeds. But under UK law, a pedal sensor is not required for the bike to be road-legal.  Note that some electric bikes use the pedal sensor to decide how much power the motor provides - they don’t have a throttle, instead a microprocessor automatically controls the motor depending on how much effort you are putting in. Heinzmann don’ do this - Heinzmann kits and bikes still have a separate throttle so you have full control, and it doesn’t matter how much effort you are putting in, only that the pedals are turning. Koga and SRAM bikes and kits do measure your pedaling effort and use this to determine how much help you get.
If you buy a kit, you are free to choose to have a pedal sensor or not as you wish. If you order a complete bike, you should have a pedal sensor unless you state in writing that you know and accept that your bike will not have a Type Approval certificate. | 
