what is error 1015 in iphone

It means you have jailbroken your phone... just google it

Read the information in the support document that you linked here to. Expand all of the information and you will find the answer to error 1015.

http://support.apple.com/kb/TS3694#error1015

Error 1015: This error is typically caused by attempts to downgrade the iPhone, iPad, or iPod touch's software. This can occur when you attempt to restore using an older .ipsw file. Downgrading to a previous version is not supported. To resolve this issue, attempt to restore with the latest iPhone, iPad, or iPod touch software available from Apple. This error can also occur when an unauthorized modification of the iOS has occurred and you are now trying to restore to an authorized, default state.