This is a charm that is used in defence against a Boggart. It only works if used together with laughter, the only measure to defeat a Boggart.
The charm together with laughter force a Boggart to assume an amusing form.
UseThe Boggart-Banishing Spell is used in Harry Potter and the Prisoner of Azkaban, Chapter 7, Moment 1 to defeat the Boggart (Professor Snape, a giant spider, a banshee, a mummy, a disembodied living hand). The screen must be refreshed until a mummy appears. After casting Riddikulus charm, by clicking on the lying mummy, the "New from J.K. Rowling" content about Boggarts is unlocked.
Its full incantation is "Riddikulus", but the letters used in the casting are: R , D, K , L, S .