Hi Jeff,
If you try to sign-in your ebay account 25 times with an incorrect password, then the next time you want to sign-in you will also have to input a 4 digit code that is displayed on the sign-in screen. This 4 digit code changes each time and is displayed as an image.
The purpose of this is to discourage hackers who use programs to auto-generate various combinations of your password. Once you do sign-in successfully the requirement to enter the 4-digit code should disappear within about 12 hours.
Michael