It just means "If you do not mark your work as copyrighted using this symbol, and later sue someone for copyright infringement, you will likely be awarded lesser damages. This is known as the “innocent infringement” defence."
U+3164 (Hangul Filler) is a Unicode character that looks empty but isn’t. It was meant for Korean syllables, but now shows up in things like odd text tricks. People use it for blank WhatsApp messages, invisible replies, and as a spacer where normal spaces don’t work. It renders blank in most places, which makes it confusing and interesting.