Sometimes you see blogs and just from looking at the first page you can read the whole body of 10 articles. Other times you see blogs and you can only read the first paragraph or so before you have to click on a title or read more link to see the rest.
The default is to see everything - but if you want to lay things out so you just get headings and a brief intro to each article on a page, then you need to use the "more" tag.
Simply type your post as you would normally, and where ever you'd like to break the piece into a new section, place your mouse cursor and click on the "more" tag from the editor.
Which technique is better?
Well somepeople like reading the whole of an article, and then the next, and then the next, without having to click anywhere. So you're quite likely to hold someone's attention for longer by displaying everything for them in one go.
However at the same time, as each post in Wordpress does also have it's own page, you're actually showing Google duplicate content because you've got that article on your site - in it's entirety - twice. So you might not want to do that (or you might want to add a duplicate content plugin).