An Ice Bong offers a special compartment for ice cubes. Often these are ice notches or an ice twist in the neck/body of the bong, where ice cubes can be placed and subsequently cool the smoke.
Cooling makes the smoke smoother and more pleasant. Furthermore, some of the bigger particles of the smoke are filtered out by the ice cubes.
If you use ice cubes it is necessary to watch the water level in the bong. On account of the ice cubes melting the level rises and may get too high for proper smoking.