Basically:
You get heat off a normal radiator from the hot water inside transferring its heat to the air outside it. If you reverse that and put the cold air inside the radiator and the hot water outside it works the other way around.
Basically like all indirect cylinder types a thermal store is just a water space with a radiator inside.
The linked boiler usually works the same way, its just a means to get the heat out of the boilers fuel into water, which then goes along a pipe and transfers into more water through a radiator or coil of pipe inside the cylinder.
