Compulsory Basic Training, or CBT, is a course — not a test you pass or fail. The aim is to make sure you can ride safely enough to practise on the road. You complete it with a DVSA-approved training body, usually in a single day.
CBT is delivered in five parts, worked through in order. You only move on when your instructor is satisfied you're ready:
Once completed, your CBT certificate (a DL196) is valid for two years. Within that time you must either pass your full motorcycle test or retake CBT to keep riding as a learner. It typically takes a day, but it's competency-based — if you need longer, you take longer.
Most learner riders need CBT before riding on the road. If you passed your car driving test you can ride a moped up to 50cc without taking CBT in some cases, but completing CBT is still strongly advised. Riders going for a full licence category will need CBT as part of the route.
CBT lets you ride on the road as a learner (with L plates, no pillion, no motorways). To get a full licence you then pass the motorcycle theory test and the practical modules. Which licence you end up with — and which bike you can ride — depends on your age and category, covered in motorcycle licence categories.
CBT and the theory test are separate. CBT is a practical course you complete; the theory test is a computer-based exam you pass. You need both on the road to a full licence.