What it is & when and how to use it.
The Sierra EGA and early VGA games were designed for VGA Mode 13h 320x200 video mode. On the commonly available video hardware of the time with tall pixels that were 20% taller than they were wide and with fewer rows of pixels, this video mode took up the entire screen giving it a 4:3 physical aspect ratio.
When displayed on modern graphics hardware with square-pixels, 4:3 screen mode (640x480) and the same number of rows of pixels, these same graphics will appear distorted (unnaturally compressed vertically to a 16:10 aspect ratio) and letterboxed with black bars above and below. The graphics do not look as the developer intended.
To preserve the original 4:3 ratio as the developer intended, the graphics need to be stretched vertically. This is the purpose of aspect correction. The aspect correction setting in DOSBox and ScummVM will extrapolate extra rows to extend the height and restore the 4:3 aspect ratio. Note that on modern wide screen monitors the game will be pillarboxed, but the game will be displayed properly.
This distortion affects all of the graphics. Note how Graham's face looks oddly roundish without the correction while aspect correction removes this distortion. The shape of his head is closer to that of a real person's face.
Note that aspect correction is not always needed and sometimes cannot be set for one reason or another. The Sierra VESA era games used 640x480 resolution (4:3) if the VESA.DRV is set for the videoDrv setting in the game's configuration file (RESOURCE.CFG) and thus aspect correction is not needed. The ScummVM team disabled aspect correction for King's Quest VI Windows version for technical reasons for the intro AVIs to play properly. Some fan made SCI games were designed for a 16:10 ratio by not using aspect correction, so these games should not have aspect correction set. (also http://sciprogramming.com/fangames.php?eng=both )
To turn on aspect correction in DOSBox, under the [render] section of the dosbox.conf set:
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For ScummVM click the "Options..." button and under the Graphics tab in the GUI check the "Aspect ratio correction" box. If you only want to set it for just one specific game, click the "Edit Game..." button go to the Graphics tab to check the "Aspect ratio correction" box.
Then enjoy your game as Sierra intended.