Electronic – What does “a.u.” mean in the scale of a graph

Actually the black "skirt" on the peripheral is to provide a sharp mask to overscan on CRT's. Judging by your photo you have excessive overscan and the corner of the beam is cut off by the "overscan skirt"

The overscan skirt is not the "shadow-mask skirt" which was designed to provide a unique aperture so that each color gun only casts its beam on the its own color phosphor on a flat screen.

Overscan skirts only apply to CRT where deflection is analog. Unlike LCD or plasma displays where pixels are addressed electrically. However LCD's may have CFL edge diffused backlights which can bloom light on the edges, so a very small edge mask is used here. For LED backlight LCD's no edge mask is required.

Bias is another word for the operating point -- a dc voltage or current about which the instantaneous value might vary.

For example, you can say you applied a "6 V peak-peak AC signal biased at +1 V". In this case the range of the signal would be from -2 to +4 V. You can see the relationship with the everyday meaning of bias, "a tendency or inclination" (dictionary.com) in this case with the meaning that although the voltage varies, it tends to be near the operating point.

As the other answers point out, the term is often used with relation to diodes and other nonlinear components.