It's a panel on the left of program's window. It contains information about readers, smart cards, keys and certificates shown in hierarchic structure. The tree shows to a user brief summary of all information. Click on particular item to see more detailed information in information window.
Tree hierarchy has 4 levels:
While moving through the tree, information shown in information window is updated and relevant menu options are also either activated or deactivated. For example if the user chooses a reader in the tree, the operations with certificates aren't available, if a key is selected it's not possible to export certificate etc.
Click with right mouse button shows context menu for some tree items. The context menu helps to perform operations quickly and easily.
Ref. also to information window.