So i finaly started playing AH, and on some monsters there is this ability - undead. I was wondering (since there is nothing in rulebook) what effect it has? Or its just affected by some cards?

Thx in advance

There are no automatic rules associated with the term "undead". It's just used by certain other cards (like the Cross common item) to trigger parts of their game effect.

There's also a Mythos environment card which increases the toughness of all undead monsters by 1.