The lock screen is the screen you see at startup and when you lock (Win+L) the PC. You will need to dismiss the lock screen to to see the sign-in screen and sign in to Windows.
By default, the sign-in screen background image has an acrylic blur effect as part of Windows 11 transparency effects.
If you would prefer to have a clear sign-in screen background image without affecting other transparency effects for users, you can enable the Show clear logon background group policy.
This tutorial will show you how to enable or disable the acrylic blur effect on the sign-in screen background image for all users in Windows 11.