Riplock is the feature of a drive that limits the speed at which DVD-Video discs are read. Its purpose is to make the drive quiet while you are watching movies. As a side effect, it also slows down the ripping of DVD video discs. The exact limited speed varies depending on the model of the drive. It is usually between 2X and 5X.
Some drives can only bypass riplock using hacked firmware. Some can do it with a software tool. With Plextor it is very easy, you just hold down the eject button as described in the manual. (There is more to it than just holding it down at any old time. Just read your manual. )
Some older drives do not have riplock at all. There are both DVDRom and DVDRw drives in existance that have riplock.