Insert Chapter Break = split an (x)html file into two physical files immediately.
Insert Chapter Marker = mark the place where you intend to physically split the (x)html file at some later time.
If you intend to leave your text in one (x)html file (regardless of how many chapters it has), you don't need to make use of either function. If you intend to split your text up into separate html files, then put your cursor at the point you want to split them (or just before) and do it immediately (chapter break) or mark it for later splitting (insert chapter marker).
Make backups and do a little experimenting. You'll get the hang of it quickly.
