Tactics is a secondary skill, that allows hero to arrange his troops on the battlefield at the beginning of combat, assuming that hero's skill level is higher than the opponent's Tactics level.
If both of the heroes have the Tactics skill, they cancel each other out, and neither of the heroes may arrange his army.
If one of the heroes have higher level of Tactics, his skill is treated being on the same level as the remainder. For example, if hero with expert Tactics encounters a hero with basic Tactics the remainder becomes 2, which means that the hero's expert level Tactics is treated as advanced level Tactis.
Tactics is one of the best secondary skills in the game.
At expert level, it allows a hero's army to advance 7 hexes, which is almost half of the battlefield's total length of 15.
This also allows hero to arrange his troops most suitable to the army's capabilities.
Furthermore, hero may take full advantage of the obstacles on the battlefield as a tactical element. Also a valid action is to "turtle" the troops, meaning that arrange the melee units to protect the ranged units. Combining Tactics with speed increasing spells like (mass) Haste or (mass) Prayer, allows even the slowest army to reach enemy lines on the first round of combat.
The downside of the skill is, that same level Tactics cancel each other out.
When heroes with the same level in the skill confront each other, neither of them may benefit from the skill, and therefore have one secondary skill less to utilise during the combat.