Codeine belongs to group of medicines called opioids. Opioids mimic the effects of naturally occurring pain reducing chemicals (endorphins) that are found in the brain and spinal cord. They act on the opioid receptors in the brain and block the transmission of pain signals. 
This combination is used to reduce fever and to relieve mild to moderate severe pain.

Paracetamol is a medicine used in the treatment of mild to moderate pain. It is also useful for reducing fever.
Caffeine increases the pulse rate and is stimulating short term, you get a kick of energy, but can be disturbing if you have an anxiety disorder.