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.

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!