Your scrapbook is like the clipboard on your computer. If you want to copy blocks and use them on other areas of your site, you can copy a block to your 'scrapbook', and then 'paste from scrapbook' once you've done so.
There are two kinds of scapbook - personal, and global. Global scapbooks are used when you'd like to have EXACTLY the same content on more than one area of your site. When you change something in your global content block, all of the instances of that block will change too. Use your global scrapbook when you'd like to keep the same across the website.
Your personal scapbook is used a little differently. You can copy and paste a block, but when you paste the block, it is a new instance. In other words, you can make changes on the block that you've pasted, only that block will change, not the original. Use your personal scapbook when you want to copy blocks, but you want them to be a little different from each other.
Using your scrapbook is easy. While in edit mode, simply click on a block that you'd like to copy, and click 'copy to scrapbook'. If you'd like the block to have multiple instances, use your personal scrapbook. If not, use your global scapbook. When you'd like to paste, click on the area below where you want to paste your block in, and select 'paste from scapbook'.
One thing that may not be obvious, is you can only paste a block into a specific block area once on a page. It's just how the programming has been done. You can use the same content on the same page on a different area, for instance, one on your main area, and one on your side block, but you can't paste in the same area more than once on each page. If you want to duplicate blocks in this way, it's better to copy and paste the actual content, rather than using scrapbook.
Another thing that's not obvious, is when you're making changes to your global scapbook, your changes will change all of your blocks. However, the changes won't show up in edit mode. You'll actually need to publish your changes to see your update take effect.
You can also view and edit your scrapbook items from your dashboard. Simply go to your dashboard, and click 'scrapbook'. Here, you can delete scrapbook items that are no longer needed.