This button refreshes the container structure from the LDAP server in the treeview panel of the main LEX browser window:
To reload the container structure, you could also press F5 when the focus is in the treeview panel or use the menu option View - Refresh.
The currently selected container in the treeview, it's parent node and it's child nodes will be refreshed. The LEX treeview cache is updated after the refresh.
So please note that this function is NOT refreshing the entire directory tree. This is because in large environments this operation could lasts quite a long time. To save time, network bandwidth and CPU cycles, only the area around the selected container is reloaded with this button. On the other hand - the refresh operation uses container as well as pseudo containers to built up the directory tree.