They are races used by trainers to give young horses racecourse experience before going on to a career over hurdles or fences. They are for horses that qualify under National Hunt rules having not run in a flat race after the age of 3 or not won a flat race after the age of 2.
They are also notorious races crackernut for trainers and owners to pull of a big priced betting coup, because horses have little previous 'form' and can be prepared quietly and turn up on race day ready. They do of course need to have ability in the first place. The Irish love their bumper races.