display calibrator

Please help, I decided to calibrate myself, but I am failing to understand what is asked.

In the sencance "Move the left slider until the brightness of the grey shape in the middle matches the backgrounds as much a possible"
Which background this is referring to?

The apple needs to match the stripes in the same box?

or the apple needs to match the space between the stripes?

or the background of the window itself?

or the apple should be as blent into the box as possible?