The second type of cards help assist the main battle in order for you to gain a better advantage. Most field cards only have arrows pointing in front of them with their attack values; Effect cards can have arrows pointing in any direction, with an effect symbolized by a letter that is explained in more detail on the card itself.
If that kinda lost you, this example card should be easier on your eyes.
This card shows multiple traits of some Effect cards - a card may have more than one kind of effect depending on where it points. This card also shows that while you harm the opponent's cards, you may also hurt one of your own Field cards if you put it right next to this one. So sometimes, you have to plan out where and how you are going to use a certain Effect card.
Pretend that this card is one that increases the Defense of the Field cards the arrows touch. It is a much wiser idea to put this card in the back row, or else you may give extra Defense to a few of the enemy cards!
Next, pretend that that card is one that drops Defense to 0. In this case, you always want to put it in the front row, or else the arrows won't be able to reach the enemy cards. However, this also means that if you place any cards on the red squares on your side, they will be hurt as well. Many Effect cards are designed like this, so you need to figure out the best way to use them.
Also, just like Arrow Pieces, any time you place an Effect card on the field, that leaves that spot open for an attack to pass right through. Is it worth it to put an Effect card in the front row when it means one less spot to attack from? Players need to balance between these types of cards carefully.