亻 is the person radical 人 in left-side form. It marks 俊 as a quality of a person - joining 仁优伟 in the family of human-attribute characters describing what kind of individual someone is.
夋 supplies the sound (qūn drifted to jùn) and a faint flavor of measured grace. The same phonetic powers 峻骏竣浚, all sharing the jun reading. The combined image is a person of stately, refined appearance - handsome and talented.
