Responders are the "owners" of the post. They are typically the creator of the post and any other people responsible for closing out the post. You can add as many responders to a post as you need, but on a typical post, there are just a few responders. However, assigning a responder is also not required. For example, if you are just making an announcement, then it may not be necessary to assign a responder to your post and you can just leave this blank.
Subscribers are the people subscribed to the post for notification purposes. Subscribers will receive notifications about all activity on the post, but are not explicitly responsible for closing the post (which is reserved for Responders). Typically, all members in the team are subscribed to the post by default, but you can add or remove people from subscribers as you see fit.