SITE123 is offering you to choose the concept of your website: you can have your visitors scroll down to the following page or be more traditional and accommodate each module in a separate page with a unique URL.
|Single Page Mode|
This is a modern trend, when the entire site is actually a very long single page divided into sections that may be reached by scrolling down or clicking menu buttons bound to anchors located down the way.
It is the "old school" method that most of us are well familiar with - each page is a separate entity with a unique web address. This option is recommended for massive websites containing numerous pages, where scrolling down can turn from simply long to tiresome.