01In plain English
You brush over part of a picture and describe what should be there, or leave it blank to remove an object. The AI fills the area so it blends with the rest of the image. Outpainting does the same beyond the edges of the frame.
02Why it matters when you are choosing
It turns fiddly retouching into a few seconds of work and is now built into many photo editors and image generators.
03What to check
Ask the vendor
Check the resolution it works at, whether results can be used commercially, and whether each fill uses up credits.