I don't know much about this ability, how do you use the grab ability? It doesn't say much in the book. What do I have to do to grapple?
From the universal monster rules out of the bestiary.
Grab (Ex) If a creature with this special attack hits with the indicated attack (usually a claw or bite attack), it deals normal damage and attempts to start a grapple as a free action without provoking an attack of opportunity. Unless otherwise noted, grab can only be used against targets of a size equal to or smaller than the creature with this ability. If the creature can use grab on creatures of other sizes, it is noted in the creature's Special Attacks line. The creature has the option to conduct the grapple normally, or simply use the part of its body it used in the grab to hold the opponent. If it chooses to do the latter, it takes a –20 penalty on its CMB check to make and maintain the grapple, but does not gain the grappled condition itself. A successful hold does not deal any extra damage unless the creature also has the constrict special attack. If the creature does not constrict, each successful grapple check it makes during successive rounds automatically deals the damage indicated for the attack that established the hold. Otherwise, it deals constriction damage as well (the amount is given in the creature's descriptive text).
Creatures with the grab special attack receive a +4 bonus on combat maneuver checks made to start and maintain a grapple.
Kolokotroni I said it doesn't say what I needed in the book. I know what it does but I don't know what to roll(Ex: A dc16 check). Can you please tell me what and who has to roll? Thanks.
The creature has to roll a CMB check, as per the regular grapple rules (in the Combat chapter of the Core Rulebook). The DC of the check is the opponent's CMD value.
If the creature hit with a normal attack using the body part with grab the creature will get a free grapple check.
So lets say we have a monster that has "Attacks: Bite (grab)". The creature makes a bite attack against the PC and hits. Roll damage as normal then roll a grapple check (creature's CMB vs PC's CMD check with any modifiers, such as the +4 from having grab). There is no DC check or saving throw; the CMB roll is all that is necessary. From there it follows the normal grapple rules from the CRB.
Kolokotroni I said it doesn't say what I needed in the book. I know what it does but I don't know what to roll(Ex: A dc16 check). Can you please tell me what and who has to roll? Thanks.
Ok, lets take as an example the bear.
"2 claws +7 (1d6+5 plus grab)"
This is part of the bears attack line. It gets grab with its claws.
When attacking, if it hits with either of it's claws, it may make a free grapple check (the bears CMB for grapple is +13). So rolls 1d20+13 vs the CMD of whatever it is hitting.