It has been covered dozens of times, 4bars covered some of it.
UK has to present it's tariff schedules to the other WTO members. At present they belong to the WTO because they are members of the EU, next year they will not be and have to join in their own right and get approved by the other members.
Being on WTO means hard borders everywhere including in Ireland. Customs checks, massive delays at ports, airports
Now if the hard Brexiters like Redwood, Mogg Johnson Fox etc get their way and reduce tariffs to say zero plus the delays in customs will completely destroy the UK manufacturing industry and they don't care.
Reducing tariffs to zero means they have to reduce the tariffs to zero for every country thus the UK will be swamped with all type of crap goods , they'll have no control.
One small example that I was actively involved in was about 15 years ago and didn't involve the UK. The USA decided to reduce tariffs to zero on a certain product that originated from China (but was also produced in the USA) to help the Chinese economy prosper. Because the Chinese product was much cheaper the Americans bought the Chinese product and the whole USA industry of that product disappeared within a few years. Only one company survived and they were our customer.