What is storehouse consciousness?

I've been reading Peter Harvey's Introduction the Buddhism and I've come across the concept of storehouse consciousness. It's in relation to Yogācāra and Chan Buddhism - originally with Yogācāra. It contains karmic seeds (not sure what they are to be fair). Can anyone give me an explanation of this concept. At the moment it seems to me to be some sort of real existence that underpins reality but I'm sure that's not it. I appreciate that there is an explanation on wikipedia about it but I'm not finding that hugely illuminating.