"High quality scan" is a paid feature which enables you to scan images larger than 8M Pixels (2592x1944) without scaling down the size. You will find the option in "Me" page---Scan--Scan Quality & File Size.

If you are a Free user, you can still process images larger than 8 megabyte pixels without checking the option "HD" (high quality scan), but the size of the image will be automatically scaled down to a maximum size allowed by CamScanner.

For paid users, images will be processed without scaling down its size. In other words, the image quality is better.