First off, what does UX/UI stand for?
In a web context, UX stands for "user experience" and refers to how a website works, while UI means "user interface" and is about how the website looks.
You may ask, why are they so important? Since a menu is just a menu and a form just a form. Well, that's not the case if you are looking to increase user engagement and reduce the drop-off rate to your website.