Post bases are hardware items designed to attach the decks support posts to the top of the concrete footing pier.  Post bases have an elevated bottom compartment with an opening that is designed to lock into a concrete anchor which is embedded in the top of the concrete pier.  The top of the post base straddles the wood support post and provides nail holes which should be filled to add strength to the connection.  The concrete anchor, post base, and support post need to come together in a relatively small area.  Planning ahead is essentia.  Make sure your framing lines up precisely with your footing locations or you're in for trouble.  An adjustable post base is an option that gives you a little more room to work with.