Psychedelic drugs are psychoactive drugs whose primary action is to alter the thought processes of the brain.
Many psychedelic drugs are thought to disable filters which block or suppress signals related to everyday functions from reaching the conscious mind.
These signals are presumed to originate in several other functions of the brain, including but not limited to the senses, emotions, memories and the unconscious (or subconscious) mind.
This effect is sometimes referred to as mind expanding, or consciousness expanding as your conscious mind becomes aware of (or sometimes assaulted by) things normally inaccessible to it.
At high levels this can overwhelm the sense of self and can result in a dissociative state.