When using Trip or Disarm – Instead of discipline the attacker must beat the target's CMD, a flat number similar to AC that must be overcome by a roll of 1d20+the attacker's Combat Maneuver Bonus (or CMB).
Each character and creature has a Combat Maneuver Defense (or CMD) that represents its ability to resist combat maneuvers. A creature’s CMD is determined using the following formula:
CMD = 10 + Base attack bonus + Strength modifier + Dexterity modifier + special size modifier + miscellaneous modifiers
A character's CMD (with all modifiers) can be viewed under the skils tab, listed as Combat Maneuver Defense.
The special size modifier for a creature’s Combat Maneuver Defense is as follows:
Fine –8, Diminutive –4, Tiny –2, Small –1, Medium +0, Large +1, Huge +2, Gargantuan +4, Colossal +8.
Some feats and abilities grant a bonus to your CMD when resisting specific maneuvers.
A creature can also add any circumstance, deflection, dodge, insight, luck, morale, profane, and sacred bonuses to AC to its CMD. Any penalties to a creature’s AC also apply to its CMD. A flat-footed creature does not add its Dexterity bonus to its CMD.
Currently we have Trip (previously knockdown) and Disarm.
If your Combat Maneuver Bonus attack roll equals or exceeds the CMD of the target, your maneuver is a success and has the listed effect. Some maneuvers have varying levels of success depending on how much your attack roll exceeds the target’s CMD. Rolling a natural 20 while attempting a combat maneuver is always a success (except when attempting to escape from bonds), while rolling a natural 1 is always a failure.