Zero Width Joiner
A Zero Width Joiner, or ZWJ for short, is a special Unicode character that "joins" multiple characters together. Depending on the font or operating system, some emojis with ZWJ's in between them will display new emojis which have something to do with the combination.
For example, a 👨 emoji + ZWJ + 🚒 will display as a firefighter emoji 👨🚒 on most up-to-date operating systems.
A combination of emojis and Zero Width Joiners are referred to as "ZWJ Sequences".