ZWJ stands for zero width joiner. It is an invisible Unicode character used between other characters to ask a platform to display them as one combined emoji.
For example, many family, couple, kiss and profession emoji are built from separate people, symbols and objects joined with ZWJ characters. The user sees one emoji, but the underlying Unicode value is a sequence.
ZWJ sequences are important because they let Unicode create meaningful emoji combinations without assigning every design a single new code point. Appearance and support can vary by platform, especially for newer sequences.