Unicode is the worldwide standard that gives every character (every letter, digit, symbol and emoji) its own number, so the same text shows up the same way on any device.
Why Unicode exists
Before Unicode, each system had its own way of numbering characters, so text written on one computer could turn into gibberish on another. Unicode fixed this by giving every character a single, permanent number called a code point, written like U+2713 for the check mark ✓.
Code points, encodings and bytes
A code point is just the number. To store or send it, a computer turns it into bytes using an encoding, almost always UTF-8 on the web. One character can take one to four bytes in UTF-8, which is why a “short” name with emoji or styled letters can still be surprisingly long.
Blocks and scripts
Related characters are grouped into blocks, such as Basic Latin, Dingbats or Mathematical Alphanumeric Symbols. The styled “fonts” you copy online are not fonts at all: they are letters from these blocks that happen to look bold, italic or cursive.
Why some characters show as boxes
If you see an empty box, the device simply does not have a font with that character. The character is still there, so copy it somewhere else and it may appear fine.
Facts to verify before publishing: the UTF-8 byte range (1–4) and the example code point U+2713.