Select Fold in the Fold menu to fold the selected lines. The first line in the selection will remain visible, while the others will be hidden. They are "folded" underneath the first line. A square with a plus symbol in it will appear to the left of the folded line. If you click it, the hidden lines will become visible again. A square with a minus will mark the first line, with a vertical line extending down from it to mark the previously folded range. Click the minus button again to refold the range.
If you did not select part of the text, and the text cursor is inside a foldable range indicated by a vertical line in the left margin, the Fold command will fold that folding range.
While folded lines are invisible, they are still fully part of the file, and still take part in all editing actions. E.g. if you select a block that includes one or more folded sections and copy the block to the clipboard with Edit|Copy, all selected lines, including lines hidden by folding, will be copied to the clipboard. Folding only affects the display. The only editing commands that take the folding into account are Edit|Delete Line, Fold|Copy Visible Lines and Fold|Delete Folded Lines.
On the Navigation page in the file type configuration, you can choose if and how EditPad Pro should add automatic folding points. Automatic folding points appear as an unfolded range that you can fold with the mouse or the Fold command.