Stratagems
A stratagem is a move you pay for to change one moment of the game.
You have CP — command points. You gain 1 every Command phase, and the ones you do not spend stay yours: your reserve grows for as long as you leave it alone.
A stratagem costs CP. You play it, it takes effect immediately, and that is that: it is not a permanent ability of your unit.
Every stratagem is written on its card: what it costs, when you can play it, and what it changes. Read the card when you play it, not before.
You never have to play one. For a first game, play none at all — you will have plenty to look at already.