Unicode characters encoded using UTF8 can technically use 1 to 4 bytes to represent a single character. However, older versions of MySQL only provided support for storing UTF8 encoded characters that used 1 to 3 bytes. This was enough to cover the most commonly used characters, but is not suitable for applications that accept user input where any character can be submitted (like emojis, which use 4 bytes). Newer versions of MySQL provide a character encoding called utf8mb4 to fix this issue.
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Brian Osborne: Adding MySQL UTF8MB4 support to hundreds of Cheap WordPress maintenance support plans 7 multi-sites
