Alt codes are the codes for entering "special" characters in PC/MS-DOS and Windows, allowing a wider character repertoire than is found on keyboards. They have been in use since the early days of the IBM PC, and have been designed for compatibility over the years, resulting in some codes from obsolete character encodings still working in addition to more modern Unicode-based ones. Codes are entered on keyboards by holding down the Alt key while pressing the appropriate digits or letters for the code.
Original DOS-based codes
The original DOS-based codes consisted of numbers from the IBM PC code pages, depending on what language setting was in effect on the PC. In English-speaking countries, it was usually CP437. Any three-digit (or less) number not starting with a zero will be interpreted as one of these codes even now, which will result in the corresponding character from the current character set if it exists.
When encodings such as Windows 1252 (and others for different languages) were implemented, the corresponding character codes could be entered by prefixing the character code point with a leading zero.
Finally, when Unicode support was added, these were done by pressing the + sign followed by a hexadecimal number. It is necessary to modify the Windows registry (create registry entry HKEY_CURRENT_USER\Control Panel\Input Method\EnableHexNumpad with type REG_SZ to value 1, and then restart or re-login to enable this setting) to allow these codes to work.