The Left Sidebar is a tool where you can find all the reference controls, including Devices, Tasks, Notes and the Browser. They are being grouped as tabs within the Left Sidebar so that you can keep them open at anytime while you are working on your project.
In order to open a certain tab in the Left Sidebar, you just need to click on it and it will get expanded. If you want to close it, click on the tab that is currently open and the Left Sidebar will automatically collapse.
Another way to open the Left Sidebar is by using the shortcut cmd + shift + ; on MacOS, or ctrl + shift + ; on Windows/Linux.
You can set the width of the opened Sidebar according to your preference.
You can use the Left Sidebar in all modes, except Zen mode (Z).