What is interlacing, and what does it do?
Just like the title says, in the GSdx settings there is a option you can change called Interlacing, and when you click the drop down menu there are a bunch of different options. Now I don't need help with using this settings or making this setting help a game run if that's what it does. I'm just curious about it and its use.

Any help is appreciated!

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Interlacing is, at this point, an old technique for video output that was implemented to compensate for the lack of bandwidth on old video connections. These days almost everything (except a lot of cable TV broadcasts, funny enough) is progressive scan, which means each video frame is drawn continuously, in horizontal rows of pixels from top to bottom of the screen. Interlaced scan, on the other hand, only sends half of the rows per frame. One frame will be odd numbered rows, the other even. So per frame, only half the picture is sent. This worked pretty damn well on old CRT TVs because not only was the resolution so bad you wouldn't notice any interlacing artifacts, but the way the picture displayed, the actual color from one frame bled away after a fraction of a second, rather than the near instantaneous color changing that our modern LCD or LED displays are capable of. Combine that with your eyes and brain's tendency to do "timestretching" and sort of fill the tiny gaps of missing light information with what it last received, and you have yourself a working display.

The PS2 used interlacing for most games because composite video out was the "default" that every console shipped with a cable for. It was capable of component output, which did have the bandwidth for progressive scan as well as improved color depth, but because component capable displays weren't quite mainstream in 2004/2005, game devs didn't really bother building their games to do progressive scan right away and we didn't start seeing it until a little bit into the consoles life, which is actually where Nintendo kind of flipped that on it's head, baking progressive scan into most of not all of their own inhouse games for the GameCube almost right at launch.

In the case of PCSX2, that setting handles deinterlacing. If a game is running in an interlaced mode, those are different methods of stitching the interlaced frames together and outputting one progressive frame. Some have bobbing, jittering, or blur issues, depending on what game it is, and that's just because they're all different methods.
I use bob as it's the only interlacing method in which it's major downside, halfed vertical resolution, can be accounted for. If you have a 1920 x 1080 monitor, and you're running an interlaced game, then have PCSX2 render a 1920 x 2160 resolution. Then there's some shaking but it's livable.
We're actually extremely lucky at Dolphin regarding the progressive scan problem. Even games that don't support progressive scan usually render in progressive scan under the hood. Hence, we have a Force Progressive Hack that works for all but ~4 or 5 GameCube games. When you consider that most GameCube games don't even support progressive scan, it's actually really nice to not have to worry about it.

Funny side-effect is that Dolphin doesn't have any interlaced support whatsoever. So in those four or five GameCube titles that actually render in Interlaced under the hood, there's literally no remedy because developers never even had to face the challenge.

If you want to see what happens when an emulator has no way to handle deinterlacing in an interlaced game, play one of the Dragon Ball Z Budokai GameCube games - then you'll understand a lot more about what deinterlacing is trying to prevent at least. PCSX2, on the other hand, has deinterlace settings that'll mitigate any shaking while Dolphin is completely clueless that anything is going wrong.

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