|MetaFilter's site and server can always use upgrades of hardware, software, and bandwidth, as well as more stable funding for continued support of its small but high-skilled moderation and backend team! If you'd like to chip in, you can donate to Metafilter.|
Once you've written a post or comment, you must preview it before it appears on MetaFilter. This gives you a chance to check over your spelling and grammar. If you are posting a comment, the preview also to see if any new comments have appeared in the thread that you are writing in that might make you want to alter your own comment. If your post or comment has any wacky HTML in it, this will show up on the preview and you can change it.
Also, the preview gives you once last chance to think about what you are posting or commenting; are you sure that's what you want to say? After all, you can't edit comments or posts that have been submitted.
You can keep on previewing as long as many times as you want (useful if you are trying to sort out wacky HTML).
Sometimes you will see someone use the phrase, "On preview, ..." in a comment. This means that after composing the text of their comment they used the preview function for a final check before posting, at which point they noticed that additional comments had been made in the thread while they were composing their comment. The text following "On preview" is an addendum meant to refine, clarify, or disambiguate the comment based on this activity that occured while the person was writing it.
Similarly, you will sometimes see a comment that begins with "On non-preview, ..." which is the opposite of the above situation. The commenter composed their comment and posted it without previewing, however during that time period other comments were made. The text following "On non-preview" is meant to address these new points, or to clarify context.