In normal settings items are distributed in a particular way, to try and help those lower down the field catch up, while those leading the race typically get weaker / less useful items. The balancing is to ensure that all players feel 'in the race' until the end.
Frantic Items strips away some of that balance and focuses on the most offensive and destructive items. With this enabled you'll see more Shells, Lightning and so on, with the idea being that races will be action packed and full of explosions, banana skins and mayhem.