As this guy says in his 7 minute youtube tutorial, thumbnails for the comic book maker are like storyboards for the filmmaker - a rough representation of what you are aiming for, a guide to producing your final, 'good copy' of the page. You might go through multiple thumbnails (another way of thinking about them would be 'drafts') before you begin pencilling the actual page, keeping in mind that those pencils are also a working document, and can be modified and erased before inking.
Here's thumbnails of pages 2 and 3 of a 5 page story called 'The Letter' by New York comics artist Jess Ruliffson,
And this one's by Isaac Cates - I'm particularly interested in that bottom tier, where he's drawn that bottom image bigger to indicate what goes in that second last, empty panel. Also the notes all around, indicating dialogue. Nice stuff.