hyperthreading allows 2 threads to be simultaneously executed, basically to be in the pipes at the same time... instead of each thread waiting its turn in line to be executed, it CAN in fact slow down some things but very few if any that are noticable, however in everyday use it also doesn't speed anything up a noticeable amount... in photoshop/video editting it gives a pretty significant increase....
hyperthreading allows a duplicate of the architectural state of the processor to split/use the same resources as the original processor... think of hyperthreading like this: if you owned one factory(cpu) and it was only using half the capacity of a warehouse(cpu resources) you also owned, you could build a second factory(duplicate processor) to utilize all your warehouse capacity/ability(cpu resources)... hope this helps, this is fundamentally how HT works.