Both popped and unpopped popcorn can be frozen, but it can affect the quality. Unpopped popcorn that has been frozen does not pop as well, and frozen popped popcorn that was opened becomes dry and stale. Popped popcorn in an airtight sealed bag should freeze fine.
Unpopped popcorn kernels have some moisture inside of them. When heated, this moisture turns to steam and causes a build up of pressure within the kernel, making it pop. If frozen, the inside of the kernel dries out, which may prevent popping. It is best to store unpopped kernels in a cool, dry cabinet for the best popping results.