An opposed check is a skill check where the DC is determined by another character's die roll, most commonly another skill roll or an attack roll. (It may at times also be more liberally used to mean any instance of the success of one roll being determined by another roll, such as the will save of terrifying rage, even though "opposed roll" is more accurate in this case.) This is in opposition to checks against a fixed DC set by the task being attempted (such as the DC to unlock a door).
An example of an opposed check is a taunt attempt (opposed by a concentration roll). The taunt is successful only if the taunt roll (taunt skill + d20) is at least the target's concentration roll (concentration skill + d20); if the taunt roll is lower, the taunt fails.