This is the organisation and prioritisation of information to communicate a message.

“Designers need to make very intentional decisions when it comes to prioritizing content. Where do you want your audience to look first? What is the call-to-action that you need to draw attention to? Hierarchy guides the user from one element to the next, ideally in a flow that corresponds to the content’s message. Also keep in mind that if you give visual importance or weight to too many elements, everything becomes flat. It’s like saying you have ten ‘favorite’ flavors of ice cream. Elements should not have to compete for our attention. Let the content shine and and make sure graphic elements don’t distract.”

Reference: https://hackdesign.org/lessons/19