The electron beam needs to get back to its starting point.
Blanking
The intensity of the electron beam during the scanning process is not constant. It varies in a logical, consistent pattern corresponding with the brightness of the picture as the beam moves across and up and down the CRT's face. If the electron beam were at full power when it returned to the beginning of a new line or field, it would illuminate phosphors on the CRT face and interfere with the information previously laid down. In order to prevent this from happening, the electron beam is turned down to a very low power so that it can return to the beginning of a new line or field. This period of time when the electron beam is turned down is called blanking.
Horizontal Blanking
In the scanning process, the electron beam scans the CRT, laying down the first line of information. When the beam reaches the edge of the area defining the TV screen, it is turned down to a low voltage, although it continues for a moment in the same direction. When the beam reaches the edge of the CRT, it quickly reverses direction and returns to the other side of the screen. Once it reaches the other side, the beam resumes its original direction. Its voltage is then turned back up to lay down another line. This process occurs every time lines are scanned.
The duration of the lowered voltage, from the end of one line to the beginning of the next, is called horizontal blanking. During this horizontal blanking period, the return of the electron beam from one side of the CRT to the other is called retrace.
Vertical Blanking
You've seen what happens at the end of each line. Something similar happens at the end of each field. After laying down a field of information, the electron beam is turned down to a low voltage before it retraces back to the top of the image. Once in position at the top of the screen, the beam's voltage is turned back up and it starts scanning a new field. The time that the beam's voltage is turned down until it is turned back up again is called vertical blanking or the vertical interval. When the electron beam is retracing back to the beginning of a new field, it's called vertical sync.
2. Vertical blanking.