Use this function to make the program resize a picture by adding/removing rectangular areas in a "content-aware" way. Content-aware resizing goes easy on content-filled, "important" parts of the picture, while deforming harder-to-notice, "unimportant" parts. This is useful when you want to change a picture's ratio of sides without having to either deform it or perform a crop that may remove important parts. Content-aware resizing resizes the picture and can change its overall ratio of sides, while leaving its important parts untouched.
To use content-aware resizing, go to Edit | Content-aware Resize [Ctrl+Shift+E].
Set a width and height and the program will analyze the picture to find its important regions, the ones that should be preserved. It then uses that analysis as a basis for choosing rectangles to add to/remove from the picture to bring it to the desired size. Quality sets the speed and precision of this analysis.
Click Mark Regions… to "inform" the program about which areas (e.g. faces or signs) should definitely keep their original proportions, and ones that may freely be distorted (e.g. skies or unbroken backgrounds), if auto-detection doesn't treat them properly.
Note that the program treats these regions only as very strong suggestions, not as an iron rule. Thus in extreme cases, when other unimportant areas have "run out," the above-mentioned deformation rectangles can lead through even the areas you have marked as important.