Convert text to Unicode escape sequences or vice versa.
Unicode is an international standard for encoding, representing, and handling text. It assigns a unique number (codepoint) to every character from every writing system, plus symbols, emojis, and control characters. Unicode aims to be the universal character set, currently containing over 149,000 characters covering 161 scripts.
Unicode codepoints can be encoded in different formats: UTF-8 (variable 1-4 bytes, ASCII-compatible), UTF-16 (2 or 4 bytes, used by JavaScript/Windows), and UTF-32 (fixed 4 bytes). UTF-8 has become the dominant encoding on the web, handling all languages while remaining efficient for ASCII text.
Unicode is the character set (mapping of characters to numbers). UTF-8 is one way to encode those numbers as bytes. Other encodings include UTF-16 and UTF-32.
This happens when your system doesn't have a font that includes that character, or when encoding is misdetected. The character exists but can't be displayed.