It is possible to explain this on a few different levels. Though obviously it will make more sense when you actually experience it. For example, if you imagine

It is possible to explain this on a few different levels. Though obviously it will make more sense when you actually experience it. For example, if you imagine the brain as a big Bayesian classifier, which is filtering and shaping the inputs through the prior "knowledge", then the psychedelics block the prior and let you experience the inputs in a different way from your sober, usual, daily experience. Now they don't completely block all the priors all the time, so you can consciously understand what's happening. Also, "inputs" don't come exclusively from the external world, they can be your thoughts, memories and so on. And without the priors blocking and shaping them, they re-surface (which is the basis of the name psychedelia) and show themselves in a new light. So you can process them again, in a better way, for example.