The phrase "top posting" was initially used to refer to old style Usenet "threaded" discussions, and email clients that repeated the entire thread of emails regarding a particular subject, and positioned the cursor for a reply at the top. Top posting in email and Usenet is still a bit of a religious issue, but the phrase was hijacked to refer to blog posts as well, where the posting mechanism and style are slightly different.
Top posting in blogging terms refers to creating a new post that is "on top" of the other, earlier posts. It's the standard way blogs work, actually, with posts listed in reverse chronological order, the newest post at the top. Top posting is rarely an issue with a blog that's individual blog, but when a blog is a community blog, with many writers posting, top posting is both an etiquette and a small technical problem.
First, the post that is on the top as the newest post is in the equivalent position of being "above the fold" in a newspaper; it attracts more attention, more readers, more visitors. It's a courtesy to other writers to wait a day before posting "on top" to allow their post to be picked up and spidered and passed around and linked.
Secondly, while Google and other search engines do spider regularly, it can take a day for a post to be spidered, depending on where and when the post is published. Consequently, it's also advisable technically to wait a day or so before top posting. At a minimum, I'd say waiting twelve hours if you've got a good reason to top post. It's a bit foolish to post a flurry of several posts in a cluster; it makes the blog look a bit spammy, and it diminishes the odds of all the posts to be picked up and linked to elsewhere.