Navigating between tree and article
"Having focus" for the tree pane means that any key that you press on your keyboard will be sent to that pane. Another way of saying this is that the tree pane "is active". Similarly, when the article pane "is active" or "has focus", anything that you type on your keyboard will be inserted into the article.
In general, "Changing focus" means that some other element than the currently active element will become active (responsive to keyboard input).
Using the mouse to change focus.
An obvious way of "setting focus to" the tree pane is to click on it with your left mouse button. After the tree pane has received focus, it will respond to your keyboard input. Similarly, clicking on the article will make the article pane "active" and anything you type on your keyboard will be inserted into the article.
Using "Ctrl Tab" to change focus
By default, you can press "Ctrl Tab" while the tree is active, and focus will shift from the tree to the article.
And when you press "Ctrl Tab" inside the article, focus will shift to the tree.
Using the "Tab" key to change focus
It is also possible to use the "Tab" key to change focus, but you need to specify this in the options screen. By default the "Tab" key inserts a tab character into the article, and "Ctrl Tab" switches focus between the tree and article. If you open the options screen, category "Shortcut keys" and enable the option "Use TAB to switch between the tree and article". pressing "Tab" inside the article will set focus to the tree and vice versa. Consequently, for inserting a tab character into the article now you need to use the combination "Ctrl Tab" instead of "Tab".