By default, the sidebar on the right is hidden. With the keyboard shortcut t r you open it. But, you can’t write in the sidebar nor drag blocks to it in this state, so what’s the use?
In the right-hand sidebar, you can open (branches of) blocks, pages, and references. Any internal link can be opened in the sidebar by holding the Shift key when clicking the link with your left mouse button (this also works to open a result from the search bar in the sidebar):
Here, I Shift-Click’ed on the Parent A link, opening it in the sidebar. In this case, the page is empty so we only see a bullet without text. However, from the sidebar we can start typing directly on the page or drag blocks onto it from the main window.
Furthermore, we get the option to close the page in the sidebar (1) and to filter the linked references (2).
To open a block in the sidebar, Shift-Click on a bullet. You can easily add child blocks to the opened block by placing your cursor in it and hitting Enter:
And those are the basics of Logseq’s right-hand sidebar! Many use it to have several sources of information open at once or to drag blocks between pages and branches.