The lock screen is the screen you see at startup and when you lock the PC. You will need to dismiss the lock screen to be able to see the sign in screen and sign in to Windows.
There is a default and user lock screen that are two completely separate lock screens in Windows 10.
The default lock screen is the one that displays at startup when secure sign-in with Ctrl+Alt+Delete is enabled.
The user lock screen is the one that you select the background image for your account in Settings, displays the last user lock screen at startup when secure sign-in with Ctrl+Alt+Delete is disabled, when a user locks the PC while signed in to Windows 10, and after the one minute idle timeout period when a user signs out of Windows 10.
This tutorial will show you how to change the default lock screen background image at startup when secure sign-in with Ctrl+Alt+Delete is enabled in Windows 10.