With your starting units, the lighter ones have less HP but have more blue attacks and lower damage values. The heavier units have more HP but usually have at maximum one blue attack and higher damage values. Try to have a balance between the two types to get chances to strike first as well as enough damage to follow through. Some units have red attack bars which indicate that they have a special ability. This can be a armor increasing shield, the ability to attack multiple times, etc. These special abilities will only activate if you land on the red space during your spin. The Respin ability is useful when you are trying to use those types of units.