What does this parking sign mean?

Hello,

When we drove in Brussels, we saw signs for parking that looked like this:
http://i.imgur.com/Nhj4X.jpg

Some streets were easy to park as they had meters, while others, had the parking sign like in the picture above, and we weren't sure what they meant, so didn't park in those street.

Thus I wondered, what does the sign indicate? Also, can you park in these areas at night or is a permit needed?