Visitor messages will be how you can keep track of why/how you earned housepoints. If you do not allow visitor messages, you can still earn housepoints and the amount will be displayed in your profile, but you will not be updated when they are added.

If you wish to activate visitor messages, click "profile" in the navibar, then go to "preferences". The third questions says "Allow visitor messages on my profile :". You will need to select "all members". Make sure you hit "save". Again, this is optional, but if we don't have permission to post, then we won't update you when you earn points.

Just as a side note, you can also use visitor messages for other posts besides housepoints, sort of like posting on someone's facebook wall ^.^