With a flowed form, as content expands, or is added, then this will be pushed across multiple pages
Each form element can be defined so that you allow a page break to occur in the middle of it i.e. does the section push to the next page, or half on one page, half on another
That particular error (actually a warning) is just stating that you have two objects, one inside another. The inner object allows page breaks, while the outer one doesn't. The warning is that the form for the inner object (text field) may not behave as you expect, because the subform does not allow page breaks