Описание тега utf8mb4

Unlike its utf8 encoding, MySQL’s utf8mb4 encoding offers full Unicode support (including four-byte UTF-8-encoded symbols).

The utf8mb4 character set is useful because nowadays we need support for storing not only language characters but also symbols, newly introduced emojis, and so on.

utf8mb4 has been available since MySQL 5.5.3 (released March 2010).

A nice read on How to support full Unicode in MySQL databases by Mathias Bynens can also shed some light on this.

More information about the utf8mb4 character set may be found on the official MySQL Documentation.