A proxy object is a Quicksilver object that is resolved dynamically, based on the current state of your computer. This is very useful if you want to create a trigger that applies to a current file/song/URL/the latest download/etc.
For example, a useful trigger might be "Current Application > Relaunch" 
Proxy objects can be added by plugins. For example, the Safari plugin's "Current Web Page" proxy object will refer to the current page you have open
You can view your available proxy objects under Quicksilver's preferences > Catalog > Quicksilver. 