Windows 10 will be arriving on July 29th and if you want to be first in the queue you can reserve your download now. Eh? No I'm not entirely sure why you want to do that rather than just download when it hits, but it's a good piece of PR for Microsoft, emphasising as it does the free nature of the upgrade.
If you're on a Windows 7 or Windows 8.x computer you should see a notification in your task bar offering the chance to reserve your download. There's no indication that the downloads are limited in number, or that you'll be prioritised for bandwidth when the downloads begin. Still, cant hurt to reserve a download can it?
And Microsoft will no doubt be letting us know how many people have already done so in the not too distant future.