Catnip is an herb that is a member of the mint family.
While calming for people,(yes you can drink catnip tea for relaxing), the natural chemicals in it give cats a bit of high feeling. They will rub in it, and sniff it until becoming almost unconscious. They may even chew it a bit. When too much has ingested, cats will sometimes foam at the mouth. It is rather scary to see.
It does no harm to the cat other than making them stagger around and look drunk.