颜值 (Yán zhí) is an online term.
颜 is found in 容颜 (rongyan) which means appearance. Therefore, the word 颜 is commonly associated appearances.
值 means value or worth.
Therefore, when it is placed together, it means face value. It is used by netizens to measure the appearances of people. In a way, with the advert of social media, it can be represented by the number of likes a person gets.
So for example, someone who is very good looking would be referred to as 颜值高 (yan zhi gao)