A psychedelic NPS is a drug which mimics the effects of drugs like LSD or magic mushrooms etc. They cause psychedelic reactions; ‘trips’, perceptual alterations and quasi-mystical experiences.
Examples are 5-MeO-DALT and the NBOMe series,

These drugs can interfere with judgement which can lead to many issues in an uncontrolled environment.
Short term risks include confusion, agression, panic and strong hallucinatory effects
Long term risks are not currently known.

psychedelics are commonly swallowed ore snorted

For more (fairly technical) information on the different psychedelics, you can visit this Wikipedia page. Be aware though, that this page is not scientifically checked.

Source: 1, 2, 3