The Olsson Block is a new hot end for your Ultimaker 2. This upgrade makes it possible to very quickly and easily change nozzles on your printer. This makes it possible to achieve much more detailed prints or considerably faster extrusion rates depending on the nozzle you choose. The upgrade can be especially suitable if you want to experiment with materials that cause more wear than normal on the nozzle, such as XT-CF20.
The standard kit comes with a 0.4mm nozzle which matches the size that the Ultimaker 2 ships with. In addition, it includes nozzles in the following sizes: 0.25mm, 0.60mm and 0.80mm. These can also be bought separately.
The standards kit comes complete with all the pieces you need except for an Allen Key which was supplied with your Ultimaker 2.
Much more information can be found in the following thread that was started by the creator Anders Olsson.