Axis is an imaginary line that is used to organize a group of elements.
When elements are arranged around an axis, the design feelings ordered. As with most thigs in life, we actually enjoy things that are ordered because they feel more stabled, comfortable, and approachable.
Although axis is an imaginary line, you can make it more apparent if the edges of surrounding elements are well defined. It seperates one thing on a vertical axis: something on the left, something on the right.
When we encounter something linear, such as an axis, we naturally follow the line in that direction.
Lines prompt movement and interactions.
If an end point isn't defined, you will follow the axis until you reach something of interest or are tired of interacting with the axis. It is popular in web design with infinite scroll. An example is Pinterest.