There are two counters to keep track of where we are on the screen. The horizontal counter counts up to 454 and then resets. The vertical counter counts up to 261 and then resets. There is a horizontal blanking interval at the start of every line, for the first 80 counts, where the HBLANK line is high. The vertical blanking interval is the first 16 lines of every frame, during which the VBLANK line is high.
The net is displayed when the horizontal counter hits 256, alternating on and off every four vertical lines. The entire screen is redrawn 60 times per second.
Next: Paddle Positioning.