Parries are moves in Tekken that let a player deflect opponent's attacking moves during their(parries') active frames and sometimes deal damage to the opponent if parry is succesfull by either automatically playing damaging animation on success or allowing players to perform attacks manually after a succesful parry.
All parries can sometimes be punished on reaction if whiffed.
Reversal
Reversal is a special type of a parry which can be chickened. If you can't chicken a parry, then it's just a parry, not a reversal.
Sabaki parry
Sabaki parry is an attack move that also has active parry frames during the startup frames of the attack moves. Sabaki parry moves can still be counterhit by attack moves if they don't parry them.