The substitute characters that will be displayed if your font doesn't know how to render that specific character (e.g. "", which you may or may not see on your device, sorry) often look like small white rectangles.
TIL that some people therefore call them "tofu" (because, well, small white rectangles), and that the name of Google's "Noto" font family is short for "no tofu", as in "we'll cover as many Unicode characters as we can".
https://en.wikipedia.org/w/index.php?title=Noto_fonts&oldid=1291815289#Etymology