Blogger call this feature Jump Break. With Jump Breaks you can show just a snippet of your post on your blog's index page. Blogger will insert a "Read more" link to the full post page where your readers can keep reading.
How to use this blogger readmore feature?
There are two ways to insert a "Read more" jump to your posts.
new post editor , you'll notice the "Insert jump break" icon in the editor's toolbar. Click this icon and the "jump break" will be inserted into your blog post at your cursor's position.
If you don't use the new post editor, you can still insert a jump break in Edit HTML mode by adding <!-- more --> where you want to break your post.
Want to change the "Read more" text to your own style? No problem. You can edit the "Read more" text by clicking Layout and then Edit the Blog Posts widget.
What if I use template with automatic readmore feature? Will discuss on the next post.
Source : Blogger Buzz