Unicode is the global standard used by most devices to display letters, numbers, symbols and other characters from hundreds of languages, run by the Unicode Consortium.
Without Unicode, the Internet would have not become international. The standard made it possible for text to have the same meaning when transferring from one device to another. Different countries used to have their own standard, making it virtually impossible to communicate with people in other locations.
Characters could be anything from ASCII (a-z,A-Z,0-9,%-$) to emojis 👫 to wingdings ❦.
Not all devices can show every Unicode character. If you see a blank box, a question mark in a box or anything similar (�), that means your device is incapable of displaying that character.