Chroma key / keying
Making one background color (usually green) transparent so you can drop any scene in behind the subject.
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What it is
The software picks a target hue, builds a transparency matte from how close each pixel sits to it, and knocks those pixels out. Green won because digital sensors carry the most detail in the green channel and human skin has almost none of it. Blue is the fallback when the subject wears green, or for blond hair and pale skin. Luma keying is the same trick run against brightness instead of hue.
Most of a good key happens before post. Light the screen evenly, within about a third of a stop across the whole surface. Keep the subject 2 to 3 meters off the screen so green does not bounce back onto their shoulders, and light them separately with a backlight to hold the edges. Shoot the least compressed format you can: 8-bit 4:2:0 footage carries quarter-resolution color, so edges break into chunky stair-steps no plugin can rescue.
Gotcha: spill. Even a clean key leaves a green rim on hair and shoulders, and the subject still reads as pasted on. Run a despill first, then add light wrap so a hint of the new background bleeds onto the subject's edges. That last step is what sells the composite.
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Key this green screen footage: pull a chroma key on the screen's hue with the matte tuned so hair edges stay soft, choking the matte no more than 1 pixel with 1 to 2 pixels of feather. Then despill to kill the green rim on skin and hair. Composite over the new background, add a light wrap sampling the background's edge color at about 15 percent opacity, and grade the subject to match the background's color temperature and black level. Finish with a light grain pass over the whole composite so both layers share the same texture.