What it looks like
Perfectly regular 50 Hz (or 60 Hz in the Americas and parts of Asia) activity, usually from one electrode with poor contact. It thickens the trace into a uniform band. 12
What it means at the bedside
The right fix is the electrode, not the notch filter. ACNS minimum standards ask for impedances below 10 kΩ, and well balanced. Turn the notch on in the viewer to see what the filter hides.