Slimming the system disk means deleting some useless contents from your C disk, which will not affect your normal use, such as some wallpapers, Japanese/Korean/traditional input methods, help files and so on.

If your hard disk capacity is tight, you can delete these contents. If your hard disk is very large, it doesn't matter whether you delete it or not, and it won't have much substantial impact on your computer operation.

360 suggests that you slim down the system, but it's just a promotion with this function in another direction.