Attack Mode is for offense, and you can click the icon on the top right. The "Attack mode" icon looks like a sword pointing to the right. If you click on it, all your units (except your miners) will go out to attack the enemy. Attack mode is the first mode you will be told to click on in Level One(Attack the Archidons.)The other two, "Defence mode" and "Garrison mode," on the other hand, you are not told to use, but you are meant to use them yourself.