Other than normal Walkers, all Heroes and weapons will be better or worse against different Walker types.
When going on a mission you'll see the portrait of your Hero and weapon. When you tap on either you’ll see a selection menu for that card type.
If a card is strong against the walkers for that mission then you'll see a yellow arrow indicating that above the weapon type icon on a card.
If the weapon or Hero is not especially strong or weak against the Walker type, there is no indicator arrow.
If the weapon or Hero is not especially strong or weak against the Walker type, there is no indicator arrow.
There's also a second yellow indicator shown in the top right corner of some cards. This indicates that the card can be used to help complete a Group challenge. Pick one of those if you want to help your Group complete the Group Board to win extra rewards!