Psychedelics are a hallucinogenic class of psychoactive drug which have the primary effect of triggering non-ordinary states of consciousness and psychedelic experiences.
Many of the known psychedelic substances have been found to bind to and activate the brain's serotonin 5-HT2A receptors. ( see Wikipedia )
Although it has been experimentally found that activating the 5-HT2A receptors leads to a marked increase in spontaneous (glutamate-mediated) synaptic activity in the prefrontal cortex, exactly how and why this leads to psychedelia remains unknown.
Further reading : Proceedings of the National Academy of Sciences