With the sidebar tab you can create custom sidebars to use throughout your pages and posts.
The default sidebar is called the "Primary Widget Area" but you can set a custom sidebar when editing the page / post.
For the sidebar names you are allowed to use uppercase, lowercase, numbers and underscores. Wordpress does not allow other characters or symbols so if you try to use them they will be stripped out.
After saving your sidebar you will see it displayed in grey beneath the button where you can delete it later.
When editing your pages and posts there will be an option called "Custom Sidebar". You can choose your custom sidebar here to use with that specific page or post.
After saving your custom sidebars you will want to add some widgets to them.
Navigate to Dashboard -> Appearance -> Widgets and you can find your new sidebars at the bottom of the list as seen in the image to the right.