Letting go to the attachment. It is tricky. Some people assume that means you just don't care about things or have any passions. That could not be further from the truth. You can be passionate about things, and care very deeply. But you also need to know that nothing ever stays the same - change is the only constant. Therefore if you attach to something or someone, things will change and you will suffer because you are attached.
Good article here (Pema is amazing):
http://www.shambhalasun.com/index.php?option=com_content&task=view&id=1610&Itemid=247
and an interesting interview with her:
http://www.beliefnet.com/story/160/story_16054_1.html