The basic page allows for easy creation of a page and adding content. In a basic page, the left-most column is reserved for the navigation. Additional widgets may be placed in this column, but will exist below the navigation element. This leaves three columns worth of space to work with.
If you just want a page with text content, then filling out the rich text editor is all that needs to be done. You can also add widgets and divide the page into different rows and column (up to three visible columns). If the rich text field is filled out and a widget is also selected via the page chosen directly below it, the widget will override and the text will not be shown.
There is a dropdown located below the page chooser for the widget (just outside of the column group). This is telling Cascade how you want the page laid out if only two columns exist in that row. The options are Left Large and Right Large, meaning the left of the two columns will span two columns worth of space, or the right of the two column with span two columns worth of space respectively (because remember, the body section of a basic page technically takes up three columns worth of space).