@thomaspolk - Have you watched these ==>>
Calibre Video Tutorials
I think its best to watch the Grand Tour as bite-size topics, and then go and 'do' whatever it was covered in the topic.
Calibre has two main modes with and without a device (Kindle, Sony Tab, Kobo Reader etc) connected.
That's what the green tick and Main are referring to, green tick indicates Book is on the device at its stored in the Device Main Memory - they wont be there is no connected device.
BR