Google's Blogger calls it a "jump break". But Raven/Raven+ calls it an "Extended Entry Marker". What it does, anyway, is to show a summary of your blog post at your Blogger blog's homepage, while providing a link, "Read more >>", to your entire post, which your audience can read in its entirety if s/he so desires.
A Bug
I always include a "jump break" in every one of my blog post. Problem is, I always do it as the last thing after composing my post (sometimes even after publishing it) mostly because of my absentmindedness.
There is this problem with the Raven/Raven+ editor - it does not enabled the 'Save' button (if it was disabled previously) after inserting a "jump break"/"Extended Entry Marker". This is rather annoying as I then have to insert a 'junk' character to enable the 'Save' button, then delete this 'junk' character before saving my now modified post. A prime target for getting this bug fixed here.