This indicates that it is legal to make a right turn at this junction when the traffic light is red. This is a privilege that is only extended to cyclists. Drivers must always wait for a green light.
You can do this at many junctions where there is a fully segregated cycle path, and there is usually no sign then. However, at junctions like this where all the traffic comes together some indication is needed to show whether a right turn is safe at this location.
The video shows the way this works. You go from one cycle path around the corner onto another without stopping, and without having to worry about motorised traffic as none of it is in your lane.
Explanatory captions on this video are visible only if you play it on a computer and not on a mobile device
The keen eyed will have noticed that there is also a white and green sign underneath the traffic light. As well as allowing right turn on red, this is also a simultaneous green junction, go to that link for more information.
Thousands of traffic light junctions across the Netherlands allow cyclists to make right turns on a red light, and this contributes to making cycle journeys faster than equivalent journeys by car.
There are several other blog posts illustrating the details of what makes Groningerstraat in Assen a good place for cycling.