Face validity is a big and fancy word that tells us if something looks like it's true or makes sense. Let's say you're playing a game with a ball and the rules say you have to throw it into a basket. If you throw the ball into the basket and it goes in, that would seem fair and right, so the rules have good face validity.
Similarly, if you're taking a math test and the questions seem related to what you learned in class, then the test has face validity. Basically, it means that something looks like it's doing what it's supposed to do, just like when you put on your shoes and they fit your feet perfectly! When something has face validity, it gives us confidence that it's accurate and trustworthy.