The leftmost pane displays a list of address directories and contact groups within each of the directories. Depending on your server configuration and installed plugins you might see multiple directories but there’s at least one directory which contains your personal contacts.
Contact groups are handy to organize your contacts. They can also be selected when composing an email message and will enter all members as a recipient. Groups will also appear in the auto-completion list when you start typing in the recipient field. A contact can be assigned to multiple groups.
Click a directory or a group in the list to show the contained contacts in the list on the right. The currently selected directory or group is highlighted in the list.