Good design makes a product understandable

If you need to give users instructions on how to complete a form, your design has failed. 

If you need to tell people the login button is at the bottom of the screen, your design has failed. 

If a user gets half way through completing a form only to be told on page four of the form that they need to upload a document – and page for of the form is the first they’re learning about that - your design has failed.