When you tell the console to draw a pixel at X, Y, you need to actually wait for the gun to be in that position to draw. These old consoles do not work like a progressive scan modern display. This is because TV's are a fixed update interval, so if these chips are synced to the amount of time it takes for the television to draw, they can just send data continuously and the TV will draw correctly.
Video game hardware is meticulously timed so that the chips inside sync up to the amount of time it takes for the television to draw lines, and when blanks occur. This period where nothing is being drawn to the screen is called a blank - Hblank = horizontal blank (aka when gun is moving right to left), Vblank = vertical blank (aka when gun is moving bottom to top). While the gun is off, nothing is being drawn to the screen. When It's back into position, it turns back on and begins drawing again. Because it has to move back to the left (or top) of the screen to begin drawing the next line, it turns off while moving right to left (or up to down) temporarily. When it's moving left to right, the gun is on and spraying phosphor onto the screen. It "scans" the image out line by line, going from the left side of the screen, to the right, and from the top of the screen, to the bottom. This process is mechanical - if you were to open up your TV and could shrink down and look inside, you'd physically see the gun moving left and right and up and down.
When the electricity touches the glass of your screen, it causes a phosphorescent glow, and that glow is what forms the basis of the image. Old televisions had what was called a phosphor gun that shows a beam of electricity to the screen of your old CRT television.
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