Four basic icons appear on slots.

A white defense icon. These give you dice to roll when your unit is defending against an attack.
A red attack icon for close combat. These give you dice to roll when your unit is attacking an adjacent enemy unit.
A blue ranged icon for ranged combat. These give you dice to roll when your unit is attacking an enemy unit from a distance.
A black and yellow movement icon. These give your unit movement points it can spend to move around the battlefield.