One of the things that a quantum computer needs to be able to do in order to function is to hold information at rest, no different from your classical computer. There are two things that make this tricky. First, the information is analog, rather than digital. Second, the environment likes to sneakily “measure” your data so that it decoheres and no longer behaves the way it should. Both kinds of problems are in practice dealt with by encoding the quantum information so that errors can be corrected.

If the word “decoheres” sounds really fancy, think about it this way: coherence versus decoherence is the difference between a rainbow and a grey cloud. In the former case the waves are able to interfere with each other in interesting ways, whereas in the latter case they scatter and do not interfere, producing boring results.