Adding meta data – read by search engines but not visible

Hello,
I just downloaded HeadSpace2 but I’m not sure if it does what I want. I was looking for an option to add with each post and/or for all posts certain meta tags. These would be the html tags that show up in the header of a page but do not display. I don’t necessarily want some of these to display but there may be certain tags that characterize all posts.
Can this be done with HeadSpace2 or is there a plugin that does this? I installed HeadSpace2 easily enough but I don’t see any of its effects, perhaps it only applies my changes to new posts.
Any tips would be appreciated,
Thanks,
Bruce