Why does the mac ask for the password 2 times?

Question

macbook

1. When rebooting, it first prompts you to select a user (me or guest)
2. I choose my profile, enter the password
3. Progress bar appears
4. Upon completion of the download, 2 fields: username and password
5. Fill out and get to the desktop

What does this mean and how to disable this "two-step" authorization?
*OS X Yosemite

Answer 1

1) when switching to yosemite, you enabled FileVault, it can be disabled in System Preferences - Security - FileVault (entire disk encryption).
2) the password for logging in under the account, you can disable it in System Preferences - Users & Groups - Login Options - Automatic Login