egg in short
egg is a meeting for highly gifted people.
Historically, it is a European offshoot of the ggg999 meeting, which was founded as a meeting for members of the Triple Nine Society® (TNS) back in 2009.
After the success of ggg999, 2011 saw the first egg (then called egg999) in London.
Highly gifted is not a well-defined term, but then 3SD above average might equally not be something everyone knows about. So if you really want to know, start with the nice PDFs on the TNS website.