Hex code
A hex code is a short code, like #8a5a2b, that names one exact colour for a screen.
In a simple example
You tell the AI to use #8a5a2b for buttons, and every button gets exactly that brown.
Why it matters to you
Words like brown are vague, while a hex code gives the same colour on every page.
Do not confuse with
A hex code names a screen colour. It does not give the printing inks of CMYK.