Hex colour
A colour written as six digits like #10B981: two each for red, green and blue.
See it in Color palette. Generate a palette, then export it as a Hex list.
In plain words
Hex is a way of counting with sixteen digits: 0 to 9, then A to F. Two hex digits cover every number from 0 to 255. A hex colour puts three such pairs in a row, for red, green and blue. So #FF0000 is full red, #000000 is black and #FFFFFF is white. Designers share hex codes because they are short and paste anywhere.