What is the line below the signal bars icon, the Wi-Fi icon, and the battery life icon?

I just updated my iPhone X to iOS 11.2 and I noticed something strange. There’s a line below the signal bars icon, the Wi-Fi icon, and the battery life icon on the lock screen in the upper right hand corner. Why is there a line below these icons? The line disappears when the phone unlocks and I swipe up from the bottom of the screen.