Lysergic acid (LSD) is a hallucinogenic drug. It causes hallucinations. An hallucination causes a person to 'see,' things that are not real. In response to seeing and hearing things that are not real people do all sorts of strange things after taking LSD. LSD is known informally as 'acid.' Somebody that has taken LSD is said to be 'on an acid trip.'
The Beatles hit, 'Lucy in the Sky with Diamonds,' is widely believed to be about an acid trip. The lyrics talk about 'kaleidoscope eyes.' A kaleidoscope is a children's toy. You put one end to your eye and look into it. If you twist the other end it changes what you see.
The author, Aldous Huxley was famous for experimenting with 'acid.'