In Raster Scan Display we just don’t focus on a particular portion of screen where a line or character would be displayed instead here we scan the entire screen. Let’s understand its working with the help of above mentioned diagram. Solid horizontal line in the diagram are called scan line.
Our Electron beam starts from left top corner(1st scan line) and continues scanning the pixels on that scan line. Frame buffer is a special memory where we store the pixel intensity values(intensity values for all screen pixels). While scanning the scan line we refer to the frame buffer and accordingly we assign color to the screen pixels. Now the electron ray/beam is kept ON while moving from left to right on scan line, but once it reaches to the right end of a scan line the electro beam is turned OFF until it reaches the left end of next scan line(This behavior is shown with dashed line in between scan lines). So moving from right to left to reach the scan line is called as horizontal retrace(Dashed Line).
We continue this behavior until we reach the last scan line of our screen, the moment we reach the right end of last scan line we try to move back to the first scan line of the screen which is termed as Vertical retrace(dotted line). During Vertical and Horizontal retrace the electron beam is OFF and while scanning the scan line it is ON.