As reported by Kotaku, the filter restricts what websites can be accessed through the system's browser (yes, the 3DS does have a browser). To unlock it so that you can view any page you wish, you'll have to connect a credit card to your account and pay 30 yen (about $.29).
This is not necessarily intended as a money-making scheme; it's instead intended to be a parental control feature. By requiring a credit card purchase, it reduces the likelihood that a child will find their way onto unsavory websites on their 3DS (though it doesn't change the fact that they may live in a home with countless other Internet-connected devices). Still, Nintendo could stand to make a decent chunk of change if enough people pay the fee; if 5 million people were to eventually do so, that would be $1.5 million before credit card companies take their cut.
There have been instances of sexual content ending up on Nintendo systems in the past, like the 3DS given as a Christmas gift that contained porn on an SD card. This won't do anything to stop anything to stop that from happening again, but Nintendo is always eager to protect children--be it from dirty websites or communicating with strangers online.