This will bring up a window presenting some data about the image. An information line will be displayed at the top or bottom of the image. Each of the red, green and blue values can range from 0 to 255. The information line also displays the colour in terms of Hue, Saturation and Value. This is often a more intuitive representation of colour.
Value represents how dark or light the colour is - say, dark red vs light red.
In this case where the image is a grey-scale image the Hue and Saturation values are 0 and the Value represents the grey level. This will convert your image to a grey-scale image with no colour. This shows how intensity values in the image are mapped to the display. You look up an original image value along the bottom axis of the graph, travel up until you hit the line, then travel left until you hit the left axis. The value on the left axis becomes the new value for that intensity.